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>

.gitea/workflows/release.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.3'
+          php-version: '8.4'
           extensions: curl, json, mbstring
           tools: composer:v2
           coverage: none