Enable `assumeImmutableResults` in group productivity-analytics

This commit solves https://gitlab.com/gitlab-org/gitlab/-/issues/338450
It enables `assumeImmutableResults` in the group productivity-analytics.

ee/app/assets/javascripts/analytics/productivity_analytics/index.js

@@ -15,7 +15,7 @@ import { getLabelsEndpoint, getMilestonesEndpoint } from './utils';
 Vue.use(VueApollo);
 
 const apolloProvider = new VueApollo({
-  defaultClient: createDefaultClient(),
+  defaultClient: createDefaultClient({}, { assumeImmutableResults: true }),
 });
 
 export default () => {