Stop managing jruby environment on ci resource runners

[donoghuc]

Currently we bake in three jruby versions in our CI runners assuming those are required in tests. This is managed in a .ruby-version file in logstash (see #17798). Maintaining the exogenous jruby versions appears to be both unnecessary and prone to busywork and errors. It should be possible to remove jruby from the test runners and ensure everything uses the jruby provided by our gradle dependency tree. After refactoring gradle to be the entrypoint to CI tasks it will be possible to manage the jruby environment as a dependency of any task that needs ruby.