Automatically deploy Review App for lib/gitlab MRs

What does this MR do and why?

Describe in detail what your merge request does and why.

Automatically deploys Review Apps in MRs with changes in lib/gitlab as part of a phased rollout to backend MRs. This should help reduce the amount of production incidents by catching QA failures early in MRs.

An example of a recent incident due to a change in lib/gitlab that would have been prevented by running Review Apps - gitlab-com/gl-infra/production#5756 (closed) (MR: !72465 (merged)).

See https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1058 for more information.

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.