Accessibility: Improvements for the Authentication Check modal dialog "Close X".

Removes the title attribute and uses a `<button>` element instead of a `<div>`.
Also simplifies the CSS and takes care of the hover/focus style.

Fixes #35142.
Built from https://develop.svn.wordpress.org/trunk@36014


git-svn-id: https://core.svn.wordpress.org/trunk@35979 1a063a9b-81f0-0310-95a4-ce76da25c4cd