[TASK] Upgrade to PHPUnit 9

Author: oliverklee

composer.json

@@ -26,7 +26,7 @@
 		"php-parallel-lint/php-parallel-lint": "^1.3.2",
 		"phing/phing": "^2.17.4",
 		"phpstan/phpstan-strict-rules": "^1.5.1",
-		"phpunit/phpunit": "^8.5.33",
+		"phpunit/phpunit": "^9.6.16",
 		"symfony/polyfill-php80": "^1.27.0"
 	},
 	"autoload": {