This officially removes support for Internet Explorer 11 in the CSS files run through the build process in Core.
Individual tickets should be opened for removing each manually maintained area of the code base targeting IE11.
The resulting changes to CSS files are included in this commit.
Fixes#53077.
Built from https://develop.svn.wordpress.org/trunk@50784
git-svn-id: http://core.svn.wordpress.org/trunk@50393 1a063a9b-81f0-0310-95a4-ce76da25c4cd
