Upgrade to Rails 8.0

[etiennebarrie]

This upgrades the Gemfile dependencies to use Rails 8.0, creates a new Gemfile for Rails 7.2, to be able to test all the currently supported Rails versions (and 6.1).

One issue we'll have is that Dependabot uses the gemspec to define the minimum Ruby version, and then can't resolve to Rails 8.0 because it requires Ruby 3.2. This is the reason we never got a PR to upgrade to Rails 8.0.

updater | 2024/11/21 15:38:59 INFO <job_920669108> Handled error whilst updating rails: dependency_file_not_resolvable {:message=>"Could not find compatible versions\n\nBecause rails >= 8.0.0.beta1 depends on Ruby >= 3.2.0\n and Gemfile depends on rails = 8.0.0,\n Ruby >= 3.2.0 is required.\nSo, because current Ruby version is = 3.1.4,\n version solving has failed."}
https://github.com/Shopify/maintenance_tasks/actions/runs/11956582146/job/33331673967

My understanding is that Dependabot will then downgrade the gem or otherwise be unable to bundle any other gems.

I think we can get around this by setting a ruby version in the Gemfile (but we don't want it to apply for the other gemfiles). But I'm curious to see it fail first, and then fix forward (#1125).