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>

gemfiles/rails_8.0_active_admin_4.x.gemfile

@@ -3,7 +3,7 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 8.0.0"
-gem "activeadmin", "4.0.0.beta16"
+gem "activeadmin", "4.0.0.beta20"
 gem "sqlite3"
 gem "concurrent-ruby", "1.3.4"
 gem "bigdecimal"