chore(deps): update dependency phpunit/phpunit to v13 [security]

Author: renovate-bot

testing/composer.json

@@ -7,7 +7,7 @@
         "google/auth": "^1.12",
         "google/cloud-tools": "dev-main",
         "guzzlehttp/guzzle": "^7.0",
-        "phpunit/phpunit": "^9.0",
+        "phpunit/phpunit": "^13.0",
         "friendsofphp/php-cs-fixer": "^3.29",
         "composer/semver": "^3.2",
         "phpstan/phpstan": "^2.0",