[Meta] Move from rake to gradle to build Logstash

We want to move from rake to Gradle to achieve the following goals:

- Be a better java citizen
- Easier to create project dependencies
- Reduce dependencies needed to build logstash
- Faster build
- Eliminate double launch of ruby.

**Steps required:**

- [x] Download JRuby with gradle instead of Rake https://github.com/elastic/logstash/issues/8234
- [ ] Download the minimal gems with gradle
- [ ] Remove the rake `test:install-core`
- [ ] Make gradle invoke _bundle install_.
- [ ] Evaluate the **rake artifact** build tasks, we have done a lot of work with it and FPM has been great so far.
- [ ] Fix environment initialization and loading https://github.com/elastic/logstash/issues/8207
- [ ] Clean unused rake task so we don't migrate them.