Switch to Rails 6 defaults

[vangberg]

We're currently on 5.1 defaults (config.load_defaults 5.1).

Todo:

• 5.2 defaults
  • https://lilyreile.medium.com/rails-5-2-new-framework-defaults-what-they-do-and-how-to-safely-uncomment-them-af931d27afc4
  • Use Rails 5.2 defaults #278
• 6.0 defaults: https://lilyreile.medium.com/rails-6-0-new-framework-defaults-what-they-do-and-how-to-safely-uncomment-them-586146f371e8
• 6.1 defaults: https://lilyreile.medium.com/rails-6-1-new-framework-defaults-what-they-do-and-how-to-safely-uncomment-them-c546b70f0c5e
• Enable Zeitwerk #268