Return Issue#iid instead of id when listing issues

app/controllers/projects/board_issues_controller.rb

@@ -7,7 +7,7 @@ class Projects::BoardIssuesController < Projects::ApplicationController
     issues = Boards::Issues::ListService.new(project, current_user, filter_params).execute
     issues = issues.page(params[:page])
 
-    render json: issues.as_json(only: [:id, :title, :confidential], include: { labels: { only: [:id, :title, :color] } })
+    render json: issues.as_json(only: [:iid, :title, :confidential], include: { labels: { only: [:id, :title, :color] } })
   end
 
   def update

spec/fixtures/api/schemas/issue.json

 {
   "type": "object",
   "required" : [
-    "id",
+    "iid",
     "title",
     "confidential"
   ],
   "properties" : {
-    "id": { "type": "integer" },
+    "iid": { "type": "integer" },
     "title": { "type": "string" },
     "confidential": { "type": "boolean" },
     "labels": {