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

8.x branch has a [`ruby-maven-libs` dependency](https://github.com/elastic/logstash/blob/cf9cf67b8b3b7e995ff076679609a1bb8aa0efa5/Gemfile.template#L12) 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.