Rails 8.1

[lcreid]

• Add Rails 8.1 to test matrix, and drop 7.1.
• Fix tests because Rails no longer generates autocomplete="off" in some helpers ([Fix #55209] Remove autocomplete="off" from hidden inputs in button_to rails/rails#55336, Allow removing autocomplete="off" for more tags rails/rails#55634).
• There may be more work to fix tests for when other PRs become part of a released Rails (e.g. Respect remove_hidden_field_autocomplete config in form builder hidden_field rails/rails#55989.