Use the gtag.js analytics library by default

[chalin]

Universal Analytics (UA) is officially deprecated:

• Data collection will stop on July 1st, 2023
• Six months after that, historical data will be gone too

The sooner projects migrate to Google Analytics 4 (GA4), the better.

As a first step in supporting projects' migration from UA to GA4, Docsy should, by default, use the gtag.js analytics library -- the only library supported by GA4 -- for all site tags.

This is a proposed concrete step in support of:

• Support Google Analytics 4 (GA4) and migration to GA4 #1096

[geriom]

PR reverted after seeing some errors. #1212

I'll take a closer look and try to figure out what went wrong.

[chalin]

In #1178 (comment), @geriom wrote:

Turns out this was not the issue, it was circumstantial evidence. I noticed the problem immediately after merging this but this did not introduce that problem. Sorry @chalin

Feel free re-submit this PR.

Ok, will do as soon as I can.