CI Matrix: Drop Rails 6, Ruby 2.7

[mamhoff]

What is the current behavior?

The CI runs fail because Ransack 4.1, which we need for Rails 7 support, is not available for Ruby < 3.0

What is the new behavior?

The CI runs do not fail, and we test newer Rubies.

Checklist

Please make sure the following requirements are complete:

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