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/support/string_clean_multiline.rb

@@ -4,7 +4,7 @@ module StringCleanMultiline
   refine String do
     def clean_multiline
       # Get rid of newlines and indentation spaces
-      strip.gsub(/\s*\n\s*/, "")
+      strip.gsub(/\s*\n\s*/, '')
     end
   end
 end