Default to Ruby 2.7.6 on local machines

What does this Merge Request do and why?

Follow up to: !2685 (merged), to allow developers to use Ruby 2.7.6 by default.

To test:

1. Check out branch.
2. Run make update (don't run gdk update during testing).
3. Use GDK as normal.

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.