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.dist.php

@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+use Behat\Config\Config;
+use Behat\Config\Extension;
+use Behat\Config\Profile;
+use Behat\Config\Suite;
+
+return (new Config())
+    ->withProfile(
+        (new Profile('default'))
+            ->withSuite(
+                (new Suite('default'))
+                    ->withPaths('%paths.base%/features')
+                    ->withContexts(\Behat\MinkExtension\Context\MinkContext::class)
+            )
+            ->withExtension(
+                new Extension(\Behat\MinkExtension\ServiceContainer\MinkExtension::class, [
+                    'base_url' => 'http://en.wikipedia.org/',
+                    'sessions' => [
+                        'default' => [
+                            'browserkit_http' => null,
+                        ],
+                    ],
+                ])
+            )
+    );