Update dependency phpunit/phpunit to v9.6.8

Author: renovate[bot]

composer.json

@@ -18,7 +18,7 @@
         "myclabs/php-enum": "^1.8"
     },
     "require-dev": {
-        "phpunit/phpunit": "9.5.23"
+        "phpunit/phpunit": "9.6.8"
     },
     "autoload": {
         "psr-4": {