Rename the MapData scalar, it conflicts with GraphQL Core Schema.

Closes #3450242

graphql/webform.base.graphqls

-scalar MapData
+scalar WebformSubmissionValue
 
 type Mutation {
   submitWebform(
@@ -50,5 +50,5 @@ input WebformSubmissionElement {
   """
   The value. Can be anything that is consumable by form API submission.
   """
-  value: MapData!
+  value: WebformSubmissionValue!
 }