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

.gitignore

@@ -12,4 +12,6 @@
 /spec/dummy/log/
 /spec/dummy/storage/
 /spec/dummy/tmp/
+tmp/
 /spec/dummy/node_modules/
+/gemfiles/