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_6.1_active_admin_2.9.gemfile

@@ -1,32 +0,0 @@
-# This file was generated by Appraisal
-
-source "https://rubygems.org"
-
-gem "rails", "~> 6.1.0"
-gem "activeadmin", "~> 2.9.0"
-gem "sqlite3", "~> 1.4"
-gem "concurrent-ruby", "1.3.4"
-gem "bigdecimal"
-gem "csv"
-gem "mutex_m"
-gem "puma"
-gem "capybara"
-gem "cuprite"
-gem "rspec_junit_formatter"
-gem "rspec-rails"
-gem "simplecov", require: false
-gem "super_diff"
-gem "fasterer"
-gem "rubocop"
-gem "rubocop-capybara"
-gem "rubocop-packaging"
-gem "rubocop-performance"
-gem "rubocop-rails"
-gem "rubocop-rspec"
-gem "rubocop-rspec_rails"
-gem "appraisal"
-gem "pry-rails"
-gem "sassc"
-gem "sprockets-rails"
-
-gemspec path: "../"

gemfiles/rails_7.0_active_admin_2.x.gemfile

@@ -1,32 +0,0 @@
-# This file was generated by Appraisal
-
-source "https://rubygems.org"
-
-gem "rails", "~> 7.0.0"
-gem "activeadmin", "~> 2.14"
-gem "sqlite3", "~> 1.4"
-gem "concurrent-ruby", "1.3.4"
-gem "bigdecimal"
-gem "csv"
-gem "mutex_m"
-gem "puma"
-gem "capybara"
-gem "cuprite"
-gem "rspec_junit_formatter"
-gem "rspec-rails"
-gem "simplecov", require: false
-gem "super_diff"
-gem "fasterer"
-gem "rubocop"
-gem "rubocop-capybara"
-gem "rubocop-packaging"
-gem "rubocop-performance"
-gem "rubocop-rails"
-gem "rubocop-rspec"
-gem "rubocop-rspec_rails"
-gem "appraisal"
-gem "pry-rails"
-gem "sassc"
-gem "sprockets-rails"
-
-gemspec path: "../"

gemfiles/rails_7.0_active_admin_3.x.gemfile

@@ -1,32 +0,0 @@
-# This file was generated by Appraisal
-
-source "https://rubygems.org"
-
-gem "rails", "~> 7.0.0"
-gem "activeadmin", "~> 3.2"
-gem "sqlite3", "~> 1.4"
-gem "concurrent-ruby", "1.3.4"
-gem "bigdecimal"
-gem "csv"
-gem "mutex_m"
-gem "puma"
-gem "capybara"
-gem "cuprite"
-gem "rspec_junit_formatter"
-gem "rspec-rails"
-gem "simplecov", require: false
-gem "super_diff"
-gem "fasterer"
-gem "rubocop"
-gem "rubocop-capybara"
-gem "rubocop-packaging"
-gem "rubocop-performance"
-gem "rubocop-rails"
-gem "rubocop-rspec"
-gem "rubocop-rspec_rails"
-gem "appraisal"
-gem "pry-rails"
-gem "sassc"
-gem "sprockets-rails"
-
-gemspec path: "../"

gemfiles/rails_7.1_active_admin_3.x.gemfile

@@ -1,32 +0,0 @@
-# This file was generated by Appraisal
-
-source "https://rubygems.org"
-
-gem "rails", "~> 7.1.0"
-gem "activeadmin", "~> 3.2"
-gem "sqlite3", "~> 1.4"
-gem "concurrent-ruby", "1.3.4"
-gem "bigdecimal"
-gem "csv"
-gem "mutex_m"
-gem "puma"
-gem "capybara"
-gem "cuprite"
-gem "rspec_junit_formatter"
-gem "rspec-rails"
-gem "simplecov", require: false
-gem "super_diff"
-gem "fasterer"
-gem "rubocop"
-gem "rubocop-capybara"
-gem "rubocop-packaging"
-gem "rubocop-performance"
-gem "rubocop-rails"
-gem "rubocop-rspec"
-gem "rubocop-rspec_rails"
-gem "appraisal"
-gem "pry-rails"
-gem "sassc"
-gem "sprockets-rails"
-
-gemspec path: "../"

gemfiles/rails_7.2_active_admin_3.x.gemfile

@@ -1,32 +0,0 @@
-# This file was generated by Appraisal
-
-source "https://rubygems.org"
-
-gem "rails", "~> 7.2.0"
-gem "activeadmin", "~> 3.2"
-gem "sqlite3"
-gem "concurrent-ruby", "1.3.4"
-gem "bigdecimal"
-gem "csv"
-gem "mutex_m"
-gem "puma"
-gem "capybara"
-gem "cuprite"
-gem "rspec_junit_formatter"
-gem "rspec-rails"
-gem "simplecov", require: false
-gem "super_diff"
-gem "fasterer"
-gem "rubocop"
-gem "rubocop-capybara"
-gem "rubocop-packaging"
-gem "rubocop-performance"
-gem "rubocop-rails"
-gem "rubocop-rspec"
-gem "rubocop-rspec_rails"
-gem "appraisal"
-gem "pry-rails"
-gem "sassc"
-gem "sprockets-rails"
-
-gemspec path: "../"

gemfiles/rails_7.2_active_admin_4.x.gemfile

@@ -3,7 +3,7 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 7.2.0"
-gem "activeadmin", "4.0.0.beta16"
+gem "activeadmin", "4.0.0.beta20"
 gem "sqlite3"
 gem "concurrent-ruby", "1.3.4"
 gem "bigdecimal"

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"