Fix CI: bump PHP requirement to ^8.4 (Symfony 8 enforces it)

CI was failing on the Install-bundle-dependencies step because
shivammathur/setup-php was installing 8.3 while Symfony 8.x dependencies
declare php >= 8.4. Local composer install worked because the dev box
runs PHP 8.5.5; CI doesn't.

Bumps:
  - framework/php/composer.json
  - framework/skeleton/symfony/composer.json
  - examples/todo/symfony/composer.json
  - .gitea/workflows/ci.yml         php-version: '8.3' → '8.4'
  - .gitea/workflows/release.yml    same
  - PLAN.md §13 Phase 1 *Detailed scope* PHP minimum row

PHPStan / cs-fixer / PHPUnit stay green locally.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/todo/symfony/composer.json

@@ -3,7 +3,7 @@
     "license": "proprietary",
     "minimum-stability": "stable",
     "require": {
-        "php": "^8.3",
+        "php": "^8.4",
         "symfony/framework-bundle": "^8.0",
         "symfony/runtime": "^8.0",
         "symfony/dotenv": "^8.0",