Use PHPUnit < 11

Author: chadicus

composer.json

@@ -12,7 +12,7 @@
         "traderinteractive/util": "^3.0||^4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": ">=6.5",
+        "phpunit/phpunit": ">=6.5 <11.0",
         "squizlabs/php_codesniffer": "^3.2"
     },
     "autoload": {