Update to Rails 8.1

[olleolleolle]

This PR upgrades to the latest, current version of Ruby on Rails, v8.1.

Release notes
Release blog post

To release this, have all the boxes ticked:

• Enable new framework defaults
• Click around the site
• When all is working, remove that file, and set 8.1 defaults in application.rb

Can be investigated after this is merged:

• Begin using Propshaft, rather than Sprockets, for asset pipeline
• Try using the built-in Markdown renderer
• Structured Logging with Rails.event

[mroderick]

I have verified the branch locally. Everything seems to work just fine

[mroderick]

When all is working, remove that file, and set 8.1 defaults in application.rb

Is that config/initializers/new_framework_defaults_8_1.rb?