dependabot: Disallow additional properties on update objects (SchemaStore#4832)

Co-authored-by: Nitsan Avni <[email protected]>

Author: JayBazuzi

src/schemas/json/dependabot-2.0.json

@@ -698,6 +698,7 @@
     },
     "update": {
       "type": "object",
+      "additionalProperties": false,
       "properties": {
         "allow": {
           "description": "Customize which updates are allowed",