Bundled Themes: Twenty Twenty add GPLv2 compatible TikTok icon to the social icon menu.

This adds a GPLv2 compatible TikTok icon to replace the non-compatible TikTok icon and update the readme.txt file. Updates previous commit which used a non-compatible TikTok icon [47589]

Props poena, audrasjb, Emlebrun.
Fixes #49893.
Built from https://develop.svn.wordpress.org/trunk@47796


git-svn-id: https://core.svn.wordpress.org/trunk@47572 1a063a9b-81f0-0310-95a4-ce76da25c4cd