Deprecate java.level

[basil]

Like jenkinsci/plugin-pom#522 but for pom rather than plugin-pom.

Deployment plan: There are only a dozen or so core components, so I plan on just making a pass through them as I upgrade to this release of pom and deleting java.level manually.

[basil]

In case you're wondering, Jesse, I am considering trying to adapt the new Initializer mojo to be callable from e.g. core and jenkins-test-harness (e.g. so that we can get the benefits of #324 not just in plugins but in core components, which we'll need to test the Insane upgrade in jenkins-test-harness for example). Anyway, I haven't gotten that far yet but this PR is the first step in that direction.

[jglick]

the benefits of #324

jenkinsci/maven-hpi-plugin#324 you mean?

[jglick]

Removing this property here should help simplify 11 migration work, but do note that core components do not follow the same logic as plugins when it comes to Java version requirements vs. the minimum Java version of core: they are upgraded by core developers, rather than by end users.

[basil]

do note that core components […] are upgraded by core developers, rather than by end users

I am a core developer who performed a large number of such upgrades recently. Did you think I did not know this?