prevent PHPUnit 10 from being installed

Since 5.2 the matthiasnoback/symfony-config-test package as a requirement
for phpunit/phpunit. As we are using PHPUnit 9.6 to run tests we need to
make sure that we do not get conflicting versions of the same package.

Author: xabbuh

composer.json

@@ -41,6 +41,7 @@
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.41",
         "matthiasnoback/symfony-dependency-injection-test": "^4.0 || ^5.0",
+        "phpunit/phpunit": "^9.6",
         "symfony/console": "^5.4 || ^6.0 || ^7.0",
         "symfony/phpunit-bridge": "^5.4 || ^6.0 || ^7.0",
         "symfony/yaml": "^5.4 || ^6.0 || ^7.0"