downgrade phpunit since it requires PHP 8.1 (#8)

Author: kasparsd

composer.json

@@ -21,7 +21,7 @@
     "dealerdirect/phpcodesniffer-composer-installer": "^1.0",
     "phpstan/phpstan": "^1.12",
     "phpcompatibility/php-compatibility": "^9.3",
-    "phpunit/phpunit": "^10.0",
+    "phpunit/phpunit": "^9.0",
     "squizlabs/php_codesniffer": "^3.7",
     "roave/security-advisories": "dev-latest"
   },