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>

.github/workflows/ci.yml

@@ -14,29 +14,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.0', '3.1', '3.2', '3.3']
+        ruby: ['3.2', '3.3', '3.4']
         gemfile:
-          - rails_6.1_active_admin_2.9
-          - rails_7.0_active_admin_2.x
-          - rails_7.0_active_admin_3.x
-          - rails_7.1_active_admin_3.x
-          - rails_7.2_active_admin_3.x
           - rails_7.2_active_admin_4.x
           - rails_8.0_active_admin_4.x
-        exclude:
-          # Rails 8 requires Ruby 3.2+
-          - ruby: '3.0'
-            gemfile: rails_8.0_active_admin_4.x
-          - ruby: '3.1'
-            gemfile: rails_8.0_active_admin_4.x
-          # Rails 7.2 requires Ruby 3.1+
-          - ruby: '3.0'
-            gemfile: rails_7.2_active_admin_3.x
-          # ActiveAdmin 4 requires Ruby 3.2+
-          - ruby: '3.0'
-            gemfile: rails_7.2_active_admin_4.x
-          - ruby: '3.1'
-            gemfile: rails_7.2_active_admin_4.x
 
     env:
       BUNDLE_GEMFILE: ${{ github.workspace }}/gemfiles/${{ matrix.gemfile }}.gemfile
@@ -68,10 +49,8 @@ jobs:
           cd spec/dummy
           # Build JavaScript with esbuild
           npm run build:js || true
-          # Build CSS with Tailwind for AA4
-          if [[ "${{ matrix.gemfile }}" == *"4.x"* ]]; then
-            npm run build:css || true
-          fi
+          # Build CSS with Tailwind
+          npm run build:css || true
 
       - name: Database setup
         run: |
@@ -98,7 +77,7 @@ jobs:
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '3.3'
+          ruby-version: '3.4'
           bundler-cache: true
 
       - name: Run RuboCop