Do not specify phpunit version

Author: laurynas

composer.json

@@ -15,7 +15,6 @@
         "php": ">=5.3.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6",
         "yoast/phpunit-polyfills": "^1.0"
     },
     "config": {