Only allow Backpack v6

Author: tabacitu

composer.json

@@ -25,7 +25,7 @@
         }
     ],
     "require": {
-        "backpack/crud": "^6.0|v6.x-dev"
+        "backpack/crud": "^6.0"
     },
     "require-dev": {
         "phpunit/phpunit" : "^9.0||^7.0",