Fix deprecated rendering of file/template

(cherry picked from commit 50965afdd642d678de4a787ccb4e4768ad5d8669)

app/controllers/concerns/requires_whitelisted_monitoring_client.rb

@@ -35,6 +35,6 @@ module RequiresWhitelistedMonitoringClient
   end
 
   def render_404
-    render file: Rails.root.join('public', '404'), layout: false, status: '404'
+    render "errors/not_found", layout: "errors", status: :not_found
   end
 end