Revert "DEP Use PHPUnit 11" (#64)

Should have been done for the new CMS 6 branch only

This reverts commit 8ad5092.

Author: GuySartorelli

composer.json

@@ -9,7 +9,7 @@
     "symfony/filesystem": "^4 || ^5 || ^6 || ^7"
     },
   "require-dev": {
-    "phpunit/phpunit": "^11.3",
+    "phpunit/phpunit": "^9.5",
     "silverstripe/framework": "^4.10",
     "squizlabs/php_codesniffer": "^3"
   },