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

spec/page_objects/shared/html_editor.rb

@@ -18,7 +18,7 @@ module Shared
     end
 
     def select_all
-      content_element.send_keys([:control, "a"])
+      content_element.send_keys([:control, 'a'])
       self
     end