Also test with Ruby compatibility matrix
This was discovered at https://gitlab.com/gitlab-org/gitlab-triage/-/issues/204 where we broke Ruby 2.4 compatibility because we didn't think of that it's only supported in Ruby 2.5+
This means we'll need to test with:
- Ruby 2.4
- Ruby 2.5
- Ruby 2.6
- Ruby 2.7
This seems a bit exceeding to me