Allow Rails 8.1

[AlistairNorman]

Summary

Rails 8.1 support!

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• I agree that my PR will be published under the same license as Solidus.
• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have localized any and all user-facing strings that I added to the source code.
• I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.50%. Comparing base (714b3ec) to head (3dde7dc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6409   +/-   ##
=======================================
  Coverage   89.50%   89.50%           
=======================================
  Files         981      981           
  Lines       20479    20479           
=======================================
  Hits        18329    18329           
  Misses       2150     2150           

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[tvdeyen]

Thanks