Bundled Themes: Update Twenty Twenty.

This brings Twenty Twenty in sync with GitHub. For a full list of changes since [46715], see https://github.com/WordPress/twentytwenty/compare/fa8c3fc...a9e73ab.

Props Anlino, karmatosed, williampatton, poena, joyously, ianbelanger, chetan200891.
See #48110, #48386, #48450, #48505, #48557.
Fixes #48571.
Built from https://develop.svn.wordpress.org/trunk@46722


git-svn-id: https://core.svn.wordpress.org/trunk@46522 1a063a9b-81f0-0310-95a4-ce76da25c4cd