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>

docs/activeadmin-tailwind-4.md

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+# ActiveAdmin + Tailwind CSS v4 Guide
+
+This guide consolidates the ActiveAdmin 4.0.0.beta19 upgrade notes with a working Tailwind v4 build setup. Use it as a template when updating other ActiveAdmin extension gems.
+
+## What Changed in ActiveAdmin 4.0.0.beta19 (from UPGRADING.md)
+
+### Template Updates
+- `_site_header.html.erb` container class changed from `sticky` to `fixed`.
+- `active_admin.html.erb` now includes the `pt-16` utility class.
+
+### Tailwind v4 Updates
+Replace the old Tailwind directives with the new v4 import and config:
+
+```diff
+-@tailwind base;
+-@tailwind components;
+-@tailwind utilities;
++@import "tailwindcss";
++
++@config "../../../tailwind-active_admin.config.mjs";
+```
+
+Update your CSS build command:
+
+```diff
+-"build:css": "tailwindcss -i ./app/assets/stylesheets/active_admin.css -o ./app/assets/builds/active_admin.css --minify -c tailwind-active_admin.config.js"
++"build:css": "npx @tailwindcss/cli -i ./app/assets/stylesheets/active_admin.css -o ./app/assets/builds/active_admin.css --minify"
+```
+
+If you use `tailwindcss-rails`, prefer the bundled `bin/tailwindcss` and the `@config` directive instead of passing `-c`.
+
+### ESM Config Warning
+Tailwind config now uses ESM. To avoid the warning:
+- Rename `tailwind-active_admin.config.js` to `tailwind-active_admin.config.mjs`, or
+- Add `"type": "module"` to `package.json`.
+
+### Breaking Changes Summary
+- jQuery and jQuery UI removed.
+- `columns` component removed (use Tailwind grid).
+- `tabs` component removed.
+- Replace `default_main_content` with `render "show_default"`.
+- Replace `as: :datepicker` with `as: :date_picker`.
+- Replace `active_admin_comments` with `active_admin_comments_for(resource)`.
+- Replace `attributes_table` with `attributes_table_for(resource)` in sidebars.
+- Removed `IndexAsBlog`, `IndexAsBlock`, `IndexAsGrid`.
+- Batch action form DSL replaced with partial support.
+- Deeply nested submenus removed (only one level supported).
+- Removed `Panel#header_action` and `index_column`.
+
+## Recommended Build Setup (tailwindcss-rails)
+
+### 1. Gemfile
+
+```ruby
+gem "activeadmin", "4.0.0.beta19"
+gem "tailwindcss-rails", "~> 4.4.0"
+```
+
+### 2. Tailwind Input
+
+```css
+/* app/assets/stylesheets/active_admin.tailwind.css */
+@import "tailwindcss";
+@config "../../../tailwind-active_admin.config.mjs";
+```
+
+### 3. Tailwind Config
+
+```javascript
+// tailwind-active_admin.config.mjs
+import { execSync } from "child_process";
+import activeAdminPlugin from "@activeadmin/activeadmin/plugin";
+
+const activeAdminPath = execSync("bundle show activeadmin", {
+  encoding: "utf-8"
+}).trim();
+
+export default {
+  content: [
+    `${activeAdminPath}/vendor/javascript/flowbite.js`,
+    `${activeAdminPath}/plugin.js`,
+    `${activeAdminPath}/app/views/**/*.{arb,erb,html,rb}`,
+    "./app/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/active_admin/**/*.{arb,erb,html,rb}",
+    "./app/views/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/layouts/active_admin*.{erb,html}",
+    "./app/javascript/**/*.js"
+  ],
+  darkMode: "selector",
+  plugins: [activeAdminPlugin]
+};
+```
+
+### 4. Build Tasks
+
+```ruby
+# lib/tasks/active_admin.rake
+namespace :active_admin do
+  desc "Build Active Admin Tailwind stylesheets"
+  task build: :environment do
+    command = [
+      Rails.root.join("bin/tailwindcss").to_s,
+      "-i", Rails.root.join("app/assets/stylesheets/active_admin.tailwind.css").to_s,
+      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
+      "-m"
+    ]
+
+    system(*command, exception: true)
+  end
+end
+```
+
+### 5. Tailwind CLI Binstub
+
+```bash
+bundle binstubs tailwindcss-ruby --force
+```
+
+## Notes for Extension Gems
+
+When updating other ActiveAdmin extension gems:
+- Keep all JS/CSS inside the gem or npm package; do not copy large files into host apps.
+- Add only a thin import to the host app (`import "your_gem"`).
+- Use Tailwind v4 `@import` + `@config` and ESM config files.
+- Avoid jQuery-based plugins or require them explicitly.