apm-agent-ruby: full coverage support uses 2.x as the latest version

[v1v]

I've just seen https://github.com/elastic/apm-integration-testing/blob/main/tests/versions/ruby.yml points to 2.x and https://github.com/elastic/apm-agent-ruby/tree/3.x and https://github.com/elastic/apm-agent-ruby/tree/4.x are branches that are not executed on a nightly basis when running the APM ITs

Actions

1. Agree whether those branches are required
2. What's the process to include a new .x branch?