Update dependency phpunit/phpunit to v8 [SECURITY]

Author: renovate[bot]

composer.json

@@ -32,7 +32,7 @@
         "guzzlehttp/psr7": "^1.8.2"
     },
     "require-dev": {
-        "phpunit/phpunit": "^4.8",
+        "phpunit/phpunit": "^8.5.52",
         "squizlabs/php_codesniffer": "~2.6",
         "friendsofphp/php-cs-fixer": "~2.12"
     },