Loosen gitlab gem requirement

What does this MR do and why?

This loosen the gitlab gem requirement to ~> 4.19 so that any 4.x version can be used. This bumps the minimal gitlab version to 4.19 as well (already in use in gitlab-org/gitlab.

This will allow to bump both gitlab-qa and gitlab_quality-test_tooling in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/122008/diffs.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.