Update dependencies and fix Rails 8.1 root setup

[tagliala]

This update was produced automatically by AI.

Also fixes the Rails 8.1 root setup: #710 pointed the rails_81 CI job at the root Gemfile but never bumped it from ~> 8.0.0, so that job silently tested Rails 8.0. The root Gemfile is now ~> 8.1.0 and the lock is refreshed. (gemfiles/rails_80 still covers 8.0.)

Finally, Rails 8.1's activesupport depends on the json gem and passes a positional options hash to JSON.parse from ActiveSupport::JSON.decode, which json 3.0 rejects (fixed by rails/rails#58601, merged on 8-1-stable but unreleased). This pins json to < 3.0 and adds a dependabot ignore rule.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.33%. Comparing base (71a291f) to head (c1a026d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #744   +/-   ##
=======================================
  Coverage   94.33%   94.33%           
=======================================
  Files          17       17           
  Lines         459      459           
=======================================
  Hits          433      433           
  Misses         26       26           

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