Use Rails branches instead of versions

Currently, our test suite for Solidus 4.1 and 4.2 break because
`concurrent-ruby` dropped a dependency on the `logger` gem, and Rails
has not released the fix yet (`require 'logger'` in the right place.

This moves the Rails version to be a Github branch rather than a
released version, in an attempt to get quicker fixes when the ecosystem
breaks.

Gemfile

@@ -8,7 +8,7 @@ gemspec require: false
 if ENV['RAILS_VERSION'] == 'master'
   gem 'rails', github: 'rails', require: false
 else
-  gem 'rails', ENV['RAILS_VERSION'] || '~> 7.0.2', require: false
+  gem 'rails', github: "rails/rails", branch: "#{ENV['RAILS_VERSION']&.tr(".", "-") || '7-0'}-stable", require: false
 end
 # rubocop:enable Bundler/DuplicatedGem