Update Ruby 2.7.2 to 2.7.4

This is part of gitlab#335890 (closed).

We now pin the ruby-2.7.2 images to just that version. Previously, updating RUBY_VERSION would mean that clients asking for ruby-2.7.2 would receive a different, newer version instead.

For future updates, I instead introduced a matcher (and the respective image job definitions) for ruby-2.7 and ruby-3.0. These tags will always contain the latest patch releases as specified in RUBY_VERSION.

This approach allows us to ship patches more easily going forward but maintains backwards compatibility with existing repos asking for 2.7.2.

Sanity checks

Looking at some of the pipeline jobs that ran:

• ruby-2.7.2 and ruby-2.7.2.patched still install 2.7.2 (not 2.7.4 as would be the case when merely changing RUBY_VERSION): https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/1512715655#L1368, https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/1512715698#L1624
• ruby-2.7.patched now installs 2.7.4: https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/1512715665#L1350
• ruby-3.0 still works as before: https://gitlab.com/gitlab-org/gitlab-build-images/-/jobs/1512715646#L1624

✅