DEP Don't include vendor-plugin as an explicit dependency (#1881)

We don't directly reference it in code or config, so we can rely on silverstripe/framework having this dependency for us.

Author: GuySartorelli

composer.json

@@ -21,8 +21,7 @@
     "require": {
         "php": "^8.3",
         "silverstripe/framework": "^6",
-        "silverstripe/versioned": "^3",
-        "silverstripe/vendor-plugin": "^2"
+        "silverstripe/versioned": "^3"
     },
     "require-dev": {
         "phpunit/phpunit": "^11.3",