Fixes for recent JRuby updates

[headius]

This PR will contain fixes for recent JRuby updates, including:

• jar-dependencies unpinning
• running jruby-head builds on appropriate Java setup
• anything else I find that needs updating

[headius]

Switching fully to Java 21 using https://github.com/actions/setup-java appears to have done the trick.

The problem here was as I suspected: Even if JRuby can find its Java with our JAVA_HOME tweaking (in setup-ruby), the system path will still point at the default Java on GHA (currently 17). By using setup-java we switch both JAVA_HOME and PATH to 21.

Most users running tests for a Ruby library against jruby-head probably won't need the command-line java or javac updated, so we'll wait and see if other projects run into this same problem.

[headius]

Supersedes changes in #3468.

[headius]

Doing a little cleanup of deprecations as well. Let me know if you want to merge as-is or can wait for a few small tweaks.

[headius]

Actually this can be merged any time. There's enough small deprecations and IntelliJ code suggestions I'll put them in a separate PR.

[headius]

Other PR for cleanup and improvements: #3478

[flavorjones]

Thank you! Looks good.