Convert Universal Analytics tracking code to Google Analytics 4

[chabala]

Google Analytics' Universal Analytics has been switched off, in favor of their new Google Analytics 4, which requires different tracking code.

While Primer allows customizing the tracker code by adding _includes/head-custom-google-analytics.html into each site's files, fixing this in the theme code will eliminate that need for most GitHub Pages users.

Remember that Primer is the default theme for GitHub Pages.

[welcome[bot]]

Welcome! Congrats on your first pull request to the Primer theme. If you haven't already, please be sure to check out the contributing guidelines.

[chabala]

@tsusdere