`<button>` elements are natively interactive, supported by any assistive technology, and must be used instead of non-semantic, non-accessible `<div>` elements.
Also, this change aligns the Media Widgets and the Customizer site icon and site logo controls with the design pattern used in the new Block Editor for similar controls.
Props ramonopoly, welcher, afercia.
Fixes#43151.
Built from https://develop.svn.wordpress.org/trunk@44796
git-svn-id: http://core.svn.wordpress.org/trunk@44628 1a063a9b-81f0-0310-95a4-ce76da25c4cd
