Specify major-minor to TargetRubyVersion for RuboCop

What does this Merge Request do and why?

Specify major-minor to TargetRubyVersion for RuboCop. You do not have to specify the revision.

Fixes up !1204 (merged)

cf. !1668 (merged) and !1290 (merged)

Merge Request checklist

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