Make Ruby 2.7.4 the default Ruby

What does this Merge Request do and why?

Prepares the GDK to make Ruby 2.7.4 the default Ruby. Part of gitlab#335890 (closed).

We will probably want to merge this together with a similar change in gitlab-build-images: gitlab-build-images!428 (merged)

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.