Update Rails to 8.1.2 and minitest to 6

[tagliala]

Ref: rails/rails#56434

Close #927

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9e9c39b) to head (838fe91).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #928   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          106       106           
=========================================
  Hits           106       106           

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[tagliala]

Issue with minitest 6 was quite subtle, since CI passed, but tests were not being executed.

This is from the log to confirm that they did rur:

Capybara starting Puma...
* Version 7.1.0, codename: Neon Witch
* Min threads: 0, max threads: 4
* Listening on http://127.0.0.1:46589/
................

Finished in 10.011381s, 1.5982 runs/s, 3.3961 assertions/s.
16 runs, 34 assertions, 0 failures, 0 errors, 0 skips
Coverage report generated for Minitest to /home/runner/work/demo.activeadmin.info/demo.activeadmin.info/coverage/coverage.xml.
Line Coverage: 100.0% (106 / 106)