Evaluate if 9.x/main branches need to keep `ruby-maven-libs` dep

[mashhurs]

8.x branch has a ruby-maven-libs dependency which is used by jar-dependency gem https://github.com/jruby/jar-dependencies/blob/3a299d6679e7bfff144c7b10d50e0b53cf14ae9e/jar-dependencies.gemspec#L33

However, this dependency got removed in 9.x/main which is the concern if plugin-manager is executed in air-gapped environment which cannot pull this dependency.

[donoghuc]

The core of the issue/concern I have here is that when logstash's pluginmanager installs a plugin which installs jars at download time we use a code path in which the jar-dependencies gem installs https://github.com/jruby/jar-dependencies/blob/3a299d6679e7bfff144c7b10d50e0b53cf14ae9e/lib/jars/maven_factory.rb#L76-L77 the ruby-maven and ruby-maven-libs (and their dependencies). Practically, these end up getting cleaned up as a side effect of the plugin manager doing cleanup of gems not activated, but none the less these gems are transitive dependences of logstash and IMO should be accounted for in our provided artifacts.