PHP: Bump the recommended version of PHP to 7.4.

This brings the recommendations in `readme.html` inline with those on https://wordpress.org/about/requirements/.

Props hareesh-pillai.
Fixes #50480.
Built from https://develop.svn.wordpress.org/trunk@48178


git-svn-id: https://core.svn.wordpress.org/trunk@47947 1a063a9b-81f0-0310-95a4-ce76da25c4cd