refactor: Modernize CI with Appraisal gem and consolidate workflows

- Add Appraisal gem for managing multiple test gemfiles
- Create test matrix for Rails 6.1-8.0 and ActiveAdmin 2.9-4.x
- Consolidate multiple workflow files into single ci.yml
- Use matrix strategy to test across Ruby 3.0-3.3
- Remove deprecated individual workflow files
- Add gemfiles/ to gitignore as they're generated
- Improve backward compatibility support

Rakefile

@@ -6,7 +6,7 @@ rescue LoadError
   puts 'You must `gem install bundler` and `bundle install` to run rake tasks'
 end
 
-APP_RAKEFILE = File.expand_path("spec/dummy/Rakefile", __dir__)
+APP_RAKEFILE = File.expand_path('spec/dummy/Rakefile', __dir__)
 load 'rails/tasks/engine.rake'
 
 load 'rails/tasks/statistics.rake'