Add a way to use ~"pipeline:run-in-ruby3" to run Ruby 3

What does this MR do and why?

• Add a way to use ~"pipeline:run-in-ruby3" to run Ruby 3 (This will stop it from running with Ruby 2.7!)
• Add a new job verify-ruby-2.7 which will always fail when we're running in Ruby 3, making sure that when we merge into the default branch, we must run it in Ruby 2.7 (so without ~"pipeline:run-in-ruby3")

Screenshots or screen recordings

Here's a random job that I picked which ran in Ruby 3 with ~"pipeline:run-in-ruby3": https://gitlab.com/gitlab-org/gitlab/-/jobs/2676692838

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.