Upgrade Rails from 8.0 to 8.1

[stefannibrasil]

Followup of #6925

Upgrade Rails from the current ~> 8.0 to the 8.1 series.

https://rubygems.org/gems/rails/versions

The previous Rails upgrade (7.1 → 8.0) was tracked in #6955

See: https://guides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-8-0-to-rails-8-1

Existing gem(s), file(s) needing updating, changing or deleting

• Gemfile — bump gem "rails", "~> 8.0" to ~> 8.1
• Gemfile.lock — regenerate via bundle update rails
• config/application.rb — review config.load_defaults
• config/environments/*.rb — review framework defaults / new config knobs
• config/initializers/new_framework_defaults_*.rb — add the 8.1 file via bin/rails app:update, then enable settings one at a time
• bin/* — bin/rails app:update may regenerate binstubs
• Any deprecation warnings surfaced when running bin/rails spec

New file(s) needing creating

• config/initializers/new_framework_defaults_1_0.rb (generated by bin/rails app:update)
• Any new config files generated by bin/rails app:update (review each carefully before committing)

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[stefannibrasil]

I am working on this 👋🏼

[stefannibrasil]

Note: merge this once #7016 has been deployed to avoid any config conflicts.