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>

Appraisals

@@ -1,64 +1,9 @@
 # frozen_string_literal: true
 
-# Rails 6.1 with ActiveAdmin 2.9
-appraise 'rails-6.1-active-admin-2.9' do
-  gem 'rails', '~> 6.1.0'
-  gem 'activeadmin', '~> 2.9.0'
-  gem 'sqlite3', '~> 1.4'
-  gem 'sassc'
-  gem 'sprockets-rails'
-  remove_gem 'propshaft'
-  remove_gem 'importmap-rails'
-end
-
-# Rails 7.0 with ActiveAdmin 2.x
-appraise 'rails-7.0-active-admin-2.x' do
-  gem 'rails', '~> 7.0.0'
-  gem 'activeadmin', '~> 2.14'
-  gem 'sqlite3', '~> 1.4'
-  gem 'sassc'
-  gem 'sprockets-rails'
-  remove_gem 'propshaft'
-  remove_gem 'importmap-rails'
-end
-
-# Rails 7.0 with ActiveAdmin 3.x
-appraise 'rails-7.0-active-admin-3.x' do
-  gem 'rails', '~> 7.0.0'
-  gem 'activeadmin', '~> 3.2'
-  gem 'sqlite3', '~> 1.4'
-  gem 'sassc'
-  gem 'sprockets-rails'
-  remove_gem 'propshaft'
-  remove_gem 'importmap-rails'
-end
-
-# Rails 7.1 with ActiveAdmin 3.x
-appraise 'rails-7.1-active-admin-3.x' do
-  gem 'rails', '~> 7.1.0'
-  gem 'activeadmin', '~> 3.2'
-  gem 'sqlite3', '~> 1.4'
-  gem 'sassc'
-  gem 'sprockets-rails'
-  remove_gem 'propshaft'
-  remove_gem 'importmap-rails'
-end
-
-# Rails 7.2 with ActiveAdmin 3.x
-appraise 'rails-7.2-active-admin-3.x' do
-  gem 'rails', '~> 7.2.0'
-  gem 'activeadmin', '~> 3.2'
-  gem 'sqlite3' # 7.2 can use newer sqlite3
-  gem 'sassc'
-  gem 'sprockets-rails'
-  remove_gem 'propshaft'
-  remove_gem 'importmap-rails'
-end
-
 # Rails 7.2 with ActiveAdmin 4.x beta
 appraise 'rails-7.2-active-admin-4.x' do
   gem 'rails', '~> 7.2.0'
-  gem 'activeadmin', '4.0.0.beta16'
+  gem 'activeadmin', '4.0.0.beta20'
   gem 'sqlite3' # 7.2 can use newer sqlite3
   gem 'importmap-rails'
   gem 'propshaft'
@@ -67,7 +12,7 @@ end
 # Rails 8.0 with ActiveAdmin 4.x beta
 appraise 'rails-8.0-active-admin-4.x' do
   gem 'rails', '~> 8.0.0'
-  gem 'activeadmin', '4.0.0.beta16'
+  gem 'activeadmin', '4.0.0.beta20'
   gem 'sqlite3' # Rails 8 needs latest sqlite3
   gem 'importmap-rails'
   gem 'propshaft'