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

Appraisals

@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+# Rails 6.1 with ActiveAdmin 2.9
+appraise 'rails-6.1-active-admin-2.9' do
+  gem 'rails', '~> 6.1.0'
+  gem 'activeadmin', '~> 2.9.0'
+  gem 'sassc'
+  gem 'sprockets-rails'
+  remove_gem 'propshaft'
+  remove_gem 'importmap-rails'
+end
+
+# Rails 7.0 with ActiveAdmin 2.x
+appraise 'rails-7.0-active-admin-2.x' do
+  gem 'rails', '~> 7.0.0'
+  gem 'activeadmin', '~> 2.14'
+  gem 'sassc'
+  gem 'sprockets-rails'
+  remove_gem 'propshaft'
+  remove_gem 'importmap-rails'
+end
+
+# Rails 7.0 with ActiveAdmin 3.x
+appraise 'rails-7.0-active-admin-3.x' do
+  gem 'rails', '~> 7.0.0'
+  gem 'activeadmin', '~> 3.2'
+  gem 'sassc'
+  gem 'sprockets-rails'
+  remove_gem 'propshaft'
+  remove_gem 'importmap-rails'
+end
+
+# Rails 7.1 with ActiveAdmin 3.x
+appraise 'rails-7.1-active-admin-3.x' do
+  gem 'rails', '~> 7.1.0'
+  gem 'activeadmin', '~> 3.2'
+  gem 'sassc'
+  gem 'sprockets-rails'
+  remove_gem 'propshaft'
+  remove_gem 'importmap-rails'
+end
+
+# Rails 7.2 with ActiveAdmin 3.x
+appraise 'rails-7.2-active-admin-3.x' do
+  gem 'rails', '~> 7.2.0'
+  gem 'activeadmin', '~> 3.2'
+  gem 'sassc'
+  gem 'sprockets-rails'
+  remove_gem 'propshaft'
+  remove_gem 'importmap-rails'
+end
+
+# Rails 7.2 with ActiveAdmin 4.x beta
+appraise 'rails-7.2-active-admin-4.x' do
+  gem 'rails', '~> 7.2.0'
+  gem 'activeadmin', '4.0.0.beta16'
+  gem 'importmap-rails'
+  gem 'propshaft'
+end
+
+# Rails 8.0 with ActiveAdmin 4.x beta
+appraise 'rails-8.0-active-admin-4.x' do
+  gem 'rails', '~> 8.0.0'
+  gem 'activeadmin', '4.0.0.beta16'
+  gem 'importmap-rails'
+  gem 'propshaft'
+end