only support Backpack v6

Author: tabacitu

composer.json

@@ -34,7 +34,7 @@
     "prefer-stable": true,
     "require": {
         "spatie/laravel-permission": "^5.0||^4.0||^3.0",
-        "backpack/crud": "^5.0|^6.0|v6.x-dev"
+        "backpack/crud": "^6.0"
     },
     "require-dev": {
         "phpunit/phpunit" : "^9.0||^7.0",