chore: Upgrade to ActiveAdmin 4.0.0.beta20 and simplify test matrix

- Update ActiveAdmin from beta16 to beta20
- Drop support for Rails 6.1, 7.0, 7.1 and ActiveAdmin 2.x/3.x
- Minimum requirements now: Rails 7.2+, Ruby 3.2+, ActiveAdmin 4.x
- Reduce CI jobs from 22+ to 6 (2 gemfiles × 3 Ruby versions)
- Clean up obsolete documentation files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/activeadmin/quill_editor/version.rb

@@ -2,7 +2,7 @@
 
 module ActiveAdmin
   module QuillEditor
-    VERSION = '2.0.0'
+    VERSION = '2.1.0'
     QUILL_VERSION = '2.0.3'
   end
 end