Document references Universal Analytics

Issue in ./help/merchandising-promotions/google-tag-manager.md

This document references Universal Analytics, whereas all customers are using GA4 now. So it would be very helpful to either update this document (i.e. the GTM Config json file) with GA4 references or provide an additional page that includes that info

Open help/merchandising-promotions/google-tag-manager.md and review where Universal Analytics is referenced. Update the document with GA4 references or determine whether an additional GA4 page is needed; done means the documentation covers the current GA4 setup for customers.

Written by the indexing model from the issue text.