fix: Improve backward compatibility for Rails 6 and Ruby 3.0+

- Lower Rails requirement from 7.0 to 6.0 for ActiveAdmin 2.9 compatibility
- Lower Ruby requirement from 3.2 to 3.0 for broader compatibility
- Update CI workflows to test with Ruby 3.0, 3.1, 3.2 where appropriate
- Maintain Ruby 3.2+ for ActiveAdmin 4 and Rails 8 tests

.github/workflows/specs_rails70.yml

@@ -13,7 +13,7 @@ jobs:
 
     strategy:
       matrix:
-        ruby: ['3.2', '3.3']
+        ruby: ['3.0', '3.1', '3.2']
 
     env:
       RAILS_VERSION: 7.0