Add Ruby 3.0, 3.1, and ruby-head to CI

[petergoldstein]

What is the current behavior?

CI only runs against Ruby 2.4 and 2.7

What is the new behavior?

CI runs against Ruby 2.4, 2.7, 3.0, 3.1, and head

To pass Rubocop against a recent version I also added the requirement that gem publishers have MFA configured on Rubygems.

Checklist

Please make sure the following requirements are complete:

• [X] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [X] All automated checks pass (CI/CD)