Define Enforcer Java version in terms of maven.compiler.release

[basil]

Alternative to #297. This matches the Maven Javadoc plugin, which uses the values of the maven.compiler.source and maven.compiler.release properties if defined. This alternative eliminates my two main objections to #297:

• It only works for Java 9+, thus eliminating the negative incentive to stay on Java 8
• It does not define a new API for consumers, thus not implicitly committing us to supporting arbitrary future Java versions via this mechanism. Rather this PR just makes it less likely that people who are using the existing Maven API will have trouble with Enforcer.

I tested this by compiling Jenkins core with -Dmaven.compiler.release=19 and -Dmaven.compiler.testRelease=19 and (after dealing with unrelated bugs) ensured that Java 19 bytecode got generated.

Closes #297.

[jglick]

It only works for Java 9+, thus eliminating the negative incentive to stay on Java 8

So who does this change help exactly? No one should be building anything in @jenkinsci currently with Java levels other than 8 or 11. Maybe I am missing something.

[MarkEWaite]

Allows the Platform SIG and others to experiment with Java 19 and beyond.

[jglick]

As in creating draft PRs which require Java 19?

[basil]

Yeah, I just did a local build with Java 19 bytecode with this PR to proactively identify areas that need work, such as jenkinsci/bridge-method-injector#29.