Set maximum PHPUnit version

Author: AdamVyborny

composer.json

@@ -32,7 +32,7 @@
         "keboola/coding-standard": "^13.0",
         "php-parallel-lint/php-parallel-lint": "^1.3",
         "phpstan/phpstan": "^1.4",
-        "phpunit/phpunit": ">=7.5",
+        "phpunit/phpunit": ">=7.5 <=9.6",
         "phpstan/phpdoc-parser": "1.5.*"
     },
     "scripts": {