Modernize PHP and Symfony version requirements; add Symfony 8 CI support

- Drop PHP <8.3 and Symfony <7.4 (both EOL), require PHP ^8.3 and symfony/config ^7.4
- Require behat ^3.31 (first version with PHP 8 attribute and PHP config support)
- Upgrade phpspec to ^8.0 (supports PHP 8.5)
- Add symfony/browser-kit and symfony/http-client to dev deps; switch from deprecated goutte driver to browserkit_http
- Update CI matrix: PHP 8.3/8.4/8.5 × Symfony 7.4 + PHP 8.4/8.5 × Symfony 8.0/8.1
- Add Symfony 8+ CI step: override behat to 4.x-dev as 3.31.0
- Restrict push trigger to master branch only (no duplicate runs on PRs)
- Add behat.dist.php for behat 4.x compatibility (PHP config format)
- Update behat.yml.dist to use FQCN extension name (works in both behat 3.x and 4.x)
- Add lock-symfony-version.sh script for pinning Symfony in CI

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

behat.yml.dist

@@ -4,8 +4,8 @@ default:
       path: "%paths.base%/features"
       contexts: [Behat\MinkExtension\Context\MinkContext]
   extensions:
-    Behat\MinkExtension:
+    Behat\MinkExtension\ServiceContainer\MinkExtension:
       base_url: http://en.wikipedia.org/
       sessions:
         default:
-          goutte: ~
+          browserkit_http: ~