Fix several build issues for JDK 16

[alexbudgh]

• Needed to bump minimum source/target versions for compiler (1.6 no
  longer supported).
• Needed to bump version of japicmp which fixes
  Error running japicmp-maven-plugin:0.14.4:cmp on Java 16-EA siom79/japicmp#275 for JDK 16.
• Needed to adjust apr path for brew dependency on Mac.

[trustin]

Thanks, @alexbudgh !

[normanmaurer]

@alexbudgh as we don't release versions of netty-tcnative-openssl-static and netty-tcnative-libressl-static you may need to add some config:

[INFO] ------------------------------------------------------------------------
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[INFO] Reactor Summary for Netty/TomcatNative [Parent] 2.0.40.Final-SNAPSHOT:
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[INFO] 
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[INFO] Netty/TomcatNative [Parent] ........................ SUCCESS [ 23.968 s]
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[INFO] Netty/TomcatNative [OpenSSL - Dynamic] ............. SUCCESS [ 29.081 s]
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[INFO] Netty/TomcatNative [OpenSSL - Static] .............. FAILURE [03:06 min]
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[INFO] Netty/TomcatNative [BoringSSL - Static] ............ SKIPPED
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[INFO] Netty/TomcatNative [LibreSSL - Static] ............. SKIPPED
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[INFO] ------------------------------------------------------------------------
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[INFO] BUILD FAILURE
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[INFO] ------------------------------------------------------------------------
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[INFO] Total time:  04:02 min
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[INFO] Finished at: 2021-06-18T09:47:23Z
30384
[INFO] ------------------------------------------------------------------------
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Error:  Failed to execute goal com.github.siom79.japicmp:japicmp-maven-plugin:0.15.3:cmp (default) on project netty-tcnative-openssl-static: Could not find previous version for artifact: io.netty:netty-tcnative-openssl-static -> [Help 1]
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Error:  
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Error:  To see the full stack trace of the errors, re-run Maven with the -e switch.
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Error:  Re-run Maven using the -X switch to enable full debug logging.
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Error:  
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Error:  For more information about the errors and possible solutions, please read the following articles:
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Error:  [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoFailureException
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Error:  
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Error:  After correcting the problems, you can resume the build with the command
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Error:    mvn <args> -rf :netty-tcnative-openssl-static
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Error: Process completed with exit code 1.

[alexbudgh]

Bah, I think there's a regression in the newer versions of japicmp:

			} else {
				throw new MojoFailureException("Could not find previous version for artifact: " + artifactVersionRange.getGroupId() + ":"
					+ artifactVersionRange.getArtifactId());
			}

They aren't checking for ignoreMissingArtifact like they do in other cases before throwing:

				if (ignoreMissingArtifact(pluginParameters, configurationVersion)) {
					getLog().warn("Could not find file, but ignoreMissingOldVersion is set tot true: " + file.getAbsolutePath());
				} else {
					throw new MojoFailureException("File '" + file.getAbsolutePath() + "' does not exist.");
				}

We are already setting

% grep -rn ignoreMissingOldVersion . --include "*.xml"
./pom.xml:236:            <ignoreMissingOldVersion>true</ignoreMissingOldVersion>

But it's not being honored...

Edit: looks like someone already reported it: siom79/japicmp#288

[alexbudgh]

Sent a PR to japicmp: siom79/japicmp#290 which seems to fix this issue for us.

[pmioulet]

FYI japicmp 0.15.4 includes the fix for the above issue. I was able to build in java 17 with it.

[normanmaurer]

This is replaced by #693