Adjust "conflic" declaration to new PHPUnit compatibility

Author: martin-helmich

composer.json

@@ -14,7 +14,7 @@
         "justinrainbow/json-schema": "^5.0"
     },
     "conflict": {
-        "phpunit/phpunit": "<6.0 || >= 8.0"
+        "phpunit/phpunit": "<8.0 || >= 9.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^8.0"