Test supported Rails versions against Ruby 4.0.0

[tagliala]

Additionally:

• Update dependencies by using the oldest supported Ruby version for
  each Gemfile
• Retest against Ruby 3.1. Since it is supported, it should be tested
  to avoid unintended breaking changes
• Lock Mocha to v2 because of Mocha 3 breaks CI #981

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.93%. Comparing base (654bcf8) to head (d5cc026).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #980   +/-   ##
=======================================
  Coverage   98.93%   98.93%           
=======================================
  Files          14       14           
  Lines         563      563           
=======================================
  Hits          557      557           
  Misses          6        6           

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