Add Java 13 support for Gradle projects

[fbricon]

Even after #1179 and #1195, gradle projects (using gradle 6) still fail to be correctly imported in Eclipse/jdt.ls.

Project configurator 'org.eclipse.buildship.configurators.base' failed to configure project 'dummy-java'
Can't convert '13' to enum type 'JavaVersion'
java.lang.IllegalArgumentException: Can't convert '13' to enum type 'JavaVersion'
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter.adaptToEnum(ProtocolToModelAdapter.java:180)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter.createView(ProtocolToModelAdapter.java:145)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter.convert(ProtocolToModelAdapter.java:204)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter.access$1500(ProtocolToModelAdapter.java:57)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter$AdaptingMethodInvoker.invoke(ProtocolToModelAdapter.java:405)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter$PropertyCachingMethodInvoker.invoke(ProtocolToModelAdapter.java:633)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter$SafeMethodInvoker.invoke(ProtocolToModelAdapter.java:670)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter$SupportedPropertyInvoker.invoke(ProtocolToModelAdapter.java:694)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter$InvocationHandlerImpl.invoke(ProtocolToModelAdapter.java:360)
	at com.sun.proxy.$Proxy54.getSourceLanguageLevel(Unknown Source)
	at org.eclipse.buildship.core.internal.util.gradle.CompatSourceSettings.getSourceLanguageLevel(CompatSourceSettings.java:41)
	at org.eclipse.buildship.core.internal.workspace.JavaSourceSettingsUpdater.update(JavaSourceSettingsUpdater.java:39)
	at org.eclipse.buildship.core.internal.workspace.BaseConfigurator.synchronizeJavaProjectInTransaction(BaseConfigurator.java:118)
	at org.eclipse.buildship.core.internal.workspace.BaseConfigurator.access$000(BaseConfigurator.java:39)
	at org.eclipse.buildship.core.internal.workspace.BaseConfigurator$1.run(BaseConfigurator.java:104)
	at org.eclipse.jdt.internal.core.BatchOperation.executeOperation(BatchOperation.java:41)
	at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:736)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2291)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2316)
	at org.eclipse.jdt.core.JavaCore.run(JavaCore.java:5862)
	at org.eclipse.jdt.core.JavaCore.run(JavaCore.java:5819)
	at org.eclipse.buildship.core.internal.workspace.BaseConfigurator.synchronizeJavaProject(BaseConfigurator.java:100)
	at org.eclipse.buildship.core.internal.workspace.BaseConfigurator.configure(BaseConfigurator.java:90)
	at org.eclipse.buildship.core.internal.workspace.BaseConfigurator.configure(BaseConfigurator.java:65)
	at org.eclipse.buildship.core.internal.extension.InternalProjectConfigurator.configure(InternalProjectConfigurator.java:64)
	at org.eclipse.buildship.core.internal.workspace.ProjectConfigurators.configureConfigurators(ProjectConfigurators.java:67)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation.synchronizeOpenWorkspaceProject(SynchronizeGradleBuildOperation.java:166)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation.synchronizeWorkspaceProject(SynchronizeGradleBuildOperation.java:140)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation.synchronizeGradleProjectWithWorkspaceProject(SynchronizeGradleBuildOperation.java:130)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation.access$000(SynchronizeGradleBuildOperation.java:45)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation$1.run(SynchronizeGradleBuildOperation.java:88)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2291)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2311)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation.synchronizeProjectsWithWorkspace(SynchronizeGradleBuildOperation.java:85)
	at org.eclipse.buildship.core.internal.workspace.SynchronizeGradleBuildOperation.run(SynchronizeGradleBuildOperation.java:67)
	at org.eclipse.buildship.core.internal.DefaultGradleBuild$SynchronizeOperation.runInToolingApi(DefaultGradleBuild.java:229)
	at org.eclipse.buildship.core.internal.operation.DefaultToolingApiOperationManager$WorkspaceRunnableAdapter.run(DefaultToolingApiOperationManager.java:57)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2291)
	at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2316)
	at org.eclipse.buildship.core.internal.operation.DefaultToolingApiOperationManager.run(DefaultToolingApiOperationManager.java:38)
	at org.eclipse.buildship.core.internal.DefaultGradleBuild$SynchronizeOperation.run(DefaultGradleBuild.java:191)
	at org.eclipse.buildship.core.internal.DefaultGradleBuild.synchronize(DefaultGradleBuild.java:99)
	at org.eclipse.buildship.core.internal.DefaultGradleBuild.synchronize(DefaultGradleBuild.java:85)
	at org.eclipse.jdt.ls.core.internal.managers.GradleBuildSupport.update(GradleBuildSupport.java:56)
	at org.eclipse.jdt.ls.core.internal.managers.ProjectsManager$3.runInWorkspace(ProjectsManager.java:465)
	at org.eclipse.core.internal.resources.InternalWorkspaceJob.run(InternalWorkspaceJob.java:42)
	at org.eclipse.core.internal.jobs.Worker.run(Worker.java:63)
Caused by: org.gradle.internal.typeconversion.TypeConversionException: Cannot convert string value 'VERSION_13' to an enum value of type 'org.gradle.api.JavaVersion' (valid case insensitive values: VERSION_1_1, VERSION_1_2, VERSION_1_3, VERSION_1_4, VERSION_1_5, VERSION_1_6, VERSION_1_7, VERSION_1_8, VERSION_1_9, VERSION_1_10, VERSION_11, VERSION_12, VERSION_HIGHER)
	at org.gradle.internal.typeconversion.EnumFromCharSequenceNotationParser.convert(EnumFromCharSequenceNotationParser.java:38)
	at org.gradle.internal.typeconversion.EnumFromCharSequenceNotationParser.convert(EnumFromCharSequenceNotationParser.java:25)
	at org.gradle.internal.typeconversion.NotationConverterToNotationParserAdapter.parseNotation(NotationConverterToNotationParserAdapter.java:31)
	at org.gradle.tooling.internal.adapter.ProtocolToModelAdapter.adaptToEnum(ProtocolToModelAdapter.java:177)
	... 46 more
Caused by: java.lang.IllegalArgumentException: Cannot convert string value 'VERSION_13' to an enum value of type 'org.gradle.api.JavaVersion' (valid case insensitive values: VERSION_1_1, VERSION_1_2, VERSION_1_3, VERSION_1_4, VERSION_1_5, VERSION_1_6, VERSION_1_7, VERSION_1_8, VERSION_1_9, VERSION_1_10, VERSION_11, VERSION_12, VERSION_HIGHER)
	at org.gradle.util.GUtil.toEnum(GUtil.java:485)
	at org.gradle.internal.typeconversion.EnumFromCharSequenceNotationParser.convert(EnumFromCharSequenceNotationParser.java:36)
	... 49 more

[JoseLion]

This seems to be solved on Buildship v3.1.3. For now only the snapshot version is available. Just add the update site to Eclipse configuration.
https://download.eclipse.org/buildship/updates/e413/snapshots/3.x