Run merge requests in Ruby 3.1 by default

What does this MR do and why?

Run merge requests in Ruby 3.1 by default

When label pipeline:run-in-ruby3_0 is provided, run in Ruby 3.0 instead.

Validation

• Without labels we're using Ruby 3.1: https://gitlab.com/gitlab-org/gitlab/-/jobs/5303292936
• With pipeline:run-in-ruby3_0 we're using Ruby 3.0: https://gitlab.com/gitlab-org/gitlab/-/jobs/5308288192

Action items before merging

• Announce this in Slack #engineering-fyi
• Announce this in Google Doc Engineering Week-in-Review

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #427446 (closed)