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

lib/activeadmin/quill_editor.rb

@@ -1,3 +1,4 @@
 # frozen_string_literal: true
 
 require 'activeadmin/quill_editor/engine'
+require 'formtastic/inputs/quill_editor_input'

lib/activeadmin/quill_editor/engine.rb

@@ -32,4 +32,4 @@ module ActiveAdmin
       end
     end
   end
-end
+end