Update PHPUnit to version 9.0

Author: ybelenko

composer.json

@@ -16,7 +16,7 @@
         "ybelenko/openapi-data-mocker-interfaces": "^1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0 || ^7.0",
+        "phpunit/phpunit": "^8.0 || ^9.0",
         "squizlabs/php_codesniffer": "^3.5"
     },
     "autoload": {