Separate Appraisal runs from primary test runs

[nickcharlton]

With Rails 8, the minimum version of Ruby becomes 3.2. Unless we drop support for prior Ruby versions (and also Rails versions), the way in which we were approaching CI can't work. Maintaining support for older Rails is not difficult because of Administrate itself, but how we approach testing so it feels unreasonable to drop support (which if we didn't test against it, we would effectively be doing) because it made CI harder to do. And we can make it work, so we will.

Previously, we ran the setup for the project as usual, and then ran setup for the Appraisals. This served two functions:

1. Make Appraisal available in the bundle,
2. Setup the database for running tests against

However, in the CI matrix, we support multiple different versions of Ruby (3.0 and up to 3.3). As we try and add Rails 8.0, this means we can't run bundle install on those versions of Ruby below 3.2 as it's a dependency constraint on Rails itself.

From experiments, we also can't:

1. Install Appraisal outside of the bundle, because it references gems in a way that Bundler is there to solve,
2. Load newer schema.rb files in older versions of Rails. Rails 7 introduced versioned schemas, and future schema versions can't be run with older versions of Rails.

Instead, what we can do is run the project setup inside a container configured to use the database in the CI environment, then manually do what Appraisal is doing to avoid issues bundling.

This container will always use the current development version of Ruby (3.2.2, at the time of this commit) taken from the .ruby-version file, in which we run just enough to setup the database and ensure the later tests can run against it.

It's unlikely that Administrate would do something which wasn't supported at the database level, but not impossible. However these tests would catch that (from experience: Rails falls back to a string on unknown types, so it's even less likely to fail).

We also need to remove the Ruby version specification from the generated Appraisal gemfiles, as otherwise these won't install. Whilst they seemed to be valid outside of this way of running things, here, they are not. We just use sed to remove it in place, as it can be discarded later and removing it would cause problems with Heroku deployment of the demo app.

Finally, we inline the previously extracted Action from #2524. The intent was to reduce duplication, but the order of operations are now so different it's not worth it and it would only be used in one place.

Extracted from #2705.