CI: Test with Ruby 4.0

[tvdeyen]

Summary

Test Ruby 4.0

Needs solidusio/solidus_auth_devise#265 main for now

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.48%. Comparing base (f8779d6) to head (219443d).
⚠️ Report is 15 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6403   +/-   ##
=======================================
  Coverage   89.48%   89.48%           
=======================================
  Files         980      980           
  Lines       20438    20438           
=======================================
  Hits        18289    18289           
  Misses       2149     2149           

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[AlistairNorman]

I fixed the errors that showed up with Ruby 4 over here #6410

[tvdeyen]

closing in favor of #6410