Remove symfony/deprecation-contracts direct dep (transitive, breaks version lock)

The lock-symfony-version.sh script pins all symfony/* packages to the
matrix version, but symfony/deprecation-contracts uses its own versioning
(3.x) rather than following Symfony's major/minor scheme. Removing it as
a direct dependency is safe — it is always pulled in transitively via
symfony/config and others.

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

composer.json

@@ -24,8 +24,7 @@
         "php": "^8.3",
         "behat/behat": "^3.31",
         "behat/mink": "^1.5",
-        "symfony/config": "^7.4",
-        "symfony/deprecation-contracts": "^3.0"
+        "symfony/config": "^7.4"
     },
     "require-dev": {
         "behat/mink-browserkit-driver": "^2.0",