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>

activeadmin-quill_editor.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |spec|
   spec.email         = 'mat@blocknot.es'
   spec.homepage      = 'https://github.com/rs-pro/activeadmin-quill_editor'
 
-  spec.required_ruby_version = '>= 3.0'
+  spec.required_ruby_version = '>= 3.2'
 
   spec.metadata['homepage_uri']    = spec.homepage
   spec.metadata['changelog_uri']   = 'https://github.com/rs-pro/activeadmin-quill_editor/blob/main/CHANGELOG.md'
@@ -25,6 +25,6 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['{app,lib,vendor}/**/*', 'LICENSE.txt', 'Rakefile', 'README.md']
   spec.require_paths = ['lib']
 
-  spec.add_dependency 'activeadmin', '>= 2.9', '< 5'
-  spec.add_dependency 'rails', '>= 6.0'
+  spec.add_dependency 'activeadmin', '>= 4.0', '< 5'
+  spec.add_dependency 'rails', '>= 7.2'
 end