Expand test matrix by Java 11

[NotMyFault]

The hosting requirements have been updated to ensure a plugin's minimum baseline tooling supports Java 11 too, alongside Java 8.

I'd like to see Java 11 added to the default Jenkinsfile, to promote Java 11 tests even further. New plugins should be tested against our Java baseline version too, by default, rather than just Java 8.

[jglick]

We prefer configurations.

This change would double the load on ci.jenkins.io for new plugins, running every test four times. That is likely overkill; it is extremely rare for a test to fail on only one of these combinations. Typically I would pick Linux on JDK 11 and Windows on JDK 8.

At this point we should consider simply setting the Jenkins baseline to 2.361.x and thus requiring Java 11 for new plugins. @basil I think that would require maven.compiler.release and maven.compiler.testRelease to be overridden, right? It does not appear that any plugin using a 2.361.x or later dep is already doing this, which confuses me.

[jglick]

I think we never followed up on jenkinsci/plugin-pom#478 (comment).

[basil]

I think we never followed up on jenkinsci/plugin-pom#478 (comment).

jenkinsci/plugin-pom#478 (comment)

jenkinsci/plugin-pom#478 (comment)

[jglick]

Scratched my head mightily until I remembered jenkinsci/maven-hpi-plugin#323! So just using a new baseline automatically turns on a Java 11 requirement. I will prepare a PR accordingly.