Fix phpunit dep

Author: LKaemmerling

composer.json

@@ -8,7 +8,7 @@
     "lkaemmerling/prometheus_client_php": "^1.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "^9.3",
+    "phpunit/phpunit": "^8.4",
     "squizlabs/php_codesniffer": "^3.5"
   },
   "license": "Apache-2.0",