Refactor epic mutations base

- Move re-used epic field to mutation base
- Move re-used arguments iid & group_path
- Update GraphQL documentation

doc/api/graphql/reference/gitlab_schema.graphql

@@ -2588,14 +2588,14 @@ input EpicSetSubscriptionInput {
   clientMutationId: String
 
   """
-  The group the epic to (un)subscribe is in
+  The group the epic to mutate is in
   """
   groupPath: ID!
 
   """
-  The iid of the epic to (un)subscribe
+  The iid of the epic to mutate
   """
-  iid: ID!
+  iid: String!
 
   """
   The desired state of the subscription

doc/api/graphql/reference/gitlab_schema.json

@@ -24409,6 +24409,20 @@
           "description": "Autogenerated input type of UpdateEpic",
           "fields": null,
           "inputFields": [
+            {
+              "name": "iid",
+              "description": "The iid of the epic to mutate",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "String",
+                  "ofType": null
+                }
+              },
+              "defaultValue": null
+            },
             {
               "name": "groupPath",
               "description": "The group the epic to mutate is in",
@@ -24519,20 +24533,6 @@
               },
               "defaultValue": null
             },
-            {
-              "name": "iid",
-              "description": "The iid of the epic to mutate",
-              "type": {
-                "kind": "NON_NULL",
-                "name": null,
-                "ofType": {
-                  "kind": "SCALAR",
-                  "name": "String",
-                  "ofType": null
-                }
-              },
-              "defaultValue": null
-            },
             {
               "name": "stateEvent",
               "description": "State event for the epic",
@@ -24863,22 +24863,22 @@
           "fields": null,
           "inputFields": [
             {
-              "name": "groupPath",
-              "description": "The group the epic to (un)subscribe is in",
+              "name": "iid",
+              "description": "The iid of the epic to mutate",
               "type": {
                 "kind": "NON_NULL",
                 "name": null,
                 "ofType": {
                   "kind": "SCALAR",
-                  "name": "ID",
+                  "name": "String",
                   "ofType": null
                 }
               },
               "defaultValue": null
             },
             {
-              "name": "iid",
-              "description": "The iid of the epic to (un)subscribe",
+              "name": "groupPath",
+              "description": "The group the epic to mutate is in",
               "type": {
                 "kind": "NON_NULL",
                 "name": null,

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

@@ -5,6 +5,19 @@ module Mutations
     class Base < ::Mutations::BaseMutation
       include Mutations::ResolvesGroup
 
+      argument :iid, GraphQL::STRING_TYPE,
+               required: true,
+               description: "The iid of the epic to mutate"
+
+      argument :group_path, GraphQL::ID_TYPE,
+               required: true,
+               description: 'The group the epic to mutate is in'
+
+      field :epic,
+            Types::EpicType,
+            null: true,
+            description: 'The epic after mutation'
+
       private
 
       def find_object(group_path:, iid:)

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

@@ -7,23 +7,10 @@ module Mutations
 
       authorize :read_epic
 
-      argument :group_path, GraphQL::ID_TYPE,
-               required: true,
-               description: 'The group the epic to (un)subscribe is in'
-
-      argument :iid, GraphQL::ID_TYPE,
-               required: true,
-               description: 'The iid of the epic to (un)subscribe'
-
       argument :subscribed_state, GraphQL::BOOLEAN_TYPE,
                required: true,
                description: 'The desired state of the subscription'
 
-      field :epic,
-            Types::EpicType,
-            null: true,
-            description: 'The epic after mutation'
-
       def resolve(args)
         group_path = args.delete(:group_path)
         epic_iid = args.delete(:iid)

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

@@ -7,10 +7,6 @@ module Mutations
 
       graphql_name 'UpdateEpic'
 
-      argument :iid, GraphQL::STRING_TYPE,
-               required: true,
-               description: "The iid of the epic to mutate"
-
       argument :state_event,
                 Types::EpicStateEventEnum,
                 required: false,
@@ -21,11 +17,6 @@ module Mutations
                required: false,
                description: 'The desired health status'
 
-      field :epic,
-            Types::EpicType,
-            null: true,
-            description: 'The epic after mutation'
-
       authorize :admin_epic
 
       def resolve(args)