chore: fix PHPUnit version to ^9.6

Author: kenjis

composer.json

@@ -37,6 +37,7 @@
         "mockery/mockery": "^1.0",
         "phpstan/extension-installer": "^1.3",
         "phpstan/phpstan-strict-rules": "^1.5",
+        "phpunit/phpunit": "^9.6",
         "rector/rector": "1.0.5"
     },
     "provide": {