Add Ruby 3 to build matrix

What does this Merge Request do and why?

Ahead of moving developers over to use Ruby 3 for general gitlab-rails development, we should make sure the GDK itself is fully compatible.

I added Ruby 3 to the build matrix and fixed some minor issues in tests, so I think we should be good in general.

Refs gitlab#356115 (closed)

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.