Understand why CI failures in 11.8 CNG were not caught by gitlab-qa

Summary

We recently shipped a release with a major bug, 11.8: https://gitlab.com/gitlab-org/gitlab-ce/issues/58073

We traced it back to some yield_self calls that were introduced, which required Ruby 2.5.3. CNG at this time was still on 2.4.

We need to understand why this was not caught during our CI pipelines, and resolve.