Update dependency phpunit/phpunit to v8 [SECURITY]

Author: renovate[bot]

composer.json

@@ -22,6 +22,6 @@
     "php": ">=7.0.0"
   },
   "require-dev": {
-    "phpunit/phpunit": "~6.0"
+    "phpunit/phpunit": "~8.5.52"
   }
 }