Rails and Ruby upgrades

[moveson]

We're a couple of Rails versions behind at the moment. It would be good to get up to Rails 7. We are also a version behind on the Heroku stack. We are using Heroku-20, but Heroku-22 is now available. Heroku-22 requires Ruby 3.1, so the upgrade path needs to reflect that ordering. The Heroku article does not indicate that Ruby 3.1 is required, but I was not able to get OST up to Heroku-22 without first upgrading to Ruby 3.1.

Path to completion:

• Bump Rails to the latest security patch (Bundle update to get the latest Rails #138)
• Upgrade Rails to 6.1 (Upgrade to Rails 6.1 #139)
• Use Rails 6.1 configs (Use Rails 6.1 config defaults #140)
• Upgrade to Ruby 3.0 (Bump Ruby to 3.0 and update bundle #146)
• Upgrade to Ruby 3.1 (Upgrade to Ruby 3.1 #151)
• Convert to Heroku-22 stack
• Upgrade to Rails 7 (Rails 7 #161)
• Use Rails 7 configs