Twenty Twenty-One uses PostCSS to convert the custom properties for Internet Explorer. Without a config file, the postcss command does nothing, causing issues in IE. This brings back the config from GitHub, which replaces all custom properties with the default values.
Props t-p, poena.
Fixes#52040.
Built from https://develop.svn.wordpress.org/trunk@49800
git-svn-id: http://core.svn.wordpress.org/trunk@49523 1a063a9b-81f0-0310-95a4-ce76da25c4cd
