Use min PHPUnit version

Author: neildaniels

composer.json

@@ -54,7 +54,7 @@
         "symfony/cache": "^4.4 || ^5 || ^6",
         "symfony/event-dispatcher": "^4.4 || ^5 || ^6",
         "phpstan/phpstan": "^1.8.1",
-        "phpunit/phpunit": "^9.3",
+        "phpunit/phpunit": "^9.6.3",
         "php-http/guzzle7-adapter": "^1.0",
         "monolog/monolog": "^2.9.1 || ^3.0",
         "php-http/cache-plugin": "^1.7",