Accessibility: Replace media placeholder clickable divs with buttons.
<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.