Change iid argument type for epics mutation

doc/api/graphql/reference/gitlab_schema.graphql

@@ -2595,7 +2595,7 @@ input EpicSetSubscriptionInput {
   """
   The iid of the epic to mutate
   """
-  iid: String!
+  iid: ID!
 
   """
   The desired state of the subscription
@@ -7799,7 +7799,7 @@ input UpdateEpicInput {
   """
   The iid of the epic to mutate
   """
-  iid: String!
+  iid: ID!
 
   """
   The IDs of labels to be removed from the epic.

doc/api/graphql/reference/gitlab_schema.json

@@ -24417,7 +24417,7 @@
                 "name": null,
                 "ofType": {
                   "kind": "SCALAR",
-                  "name": "String",
+                  "name": "ID",
                   "ofType": null
                 }
               },
@@ -24870,7 +24870,7 @@
                 "name": null,
                 "ofType": {
                   "kind": "SCALAR",
-                  "name": "String",
+                  "name": "ID",
                   "ofType": null
                 }
               },

ee/app/graphql/mutations/epics/base.rb

@@ -5,7 +5,7 @@ module Mutations
     class Base < ::Mutations::BaseMutation
       include Mutations::ResolvesGroup
 
-      argument :iid, GraphQL::STRING_TYPE,
+      argument :iid, GraphQL::ID_TYPE,
                required: true,
                description: "The iid of the epic to mutate"