Check removals marked for 15.0 and ensure they have deprecation warnings in place

We should check GitLab codebases (at least omnibus-gitlab, Charts, and gitlab-org/gitlab ones) and see that everything that is marked for removal in %15.0 which requires changes in omnibus-gitlab or Charts have deprecation warnings in place to inform users. This might a cross-team effort, but I think Distribution should be driving it as omnibus-gitlab and Charts might be the closes points-of-contacts for administrators.