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 gitlab!122008 (diffs).

MR acceptance checklist

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