Update google analytics README regarding G- and UA- tags

[domsleee]

G'day

I think we should update the README to reflect a few things:

1. Google Analytics 4 tags (G- tags) are preferred over Universal Analytics (UA-) tags, and the google-analytics.html snippet used in this theme works for both types
2. The Universal Analytics (UA-) tags will stop being processed in July 2023

This information would save the next developer a few minutes of finding out if the theme supports G- tags or not, so I think it's worthwhile.