Accessibility: Remove the title attribute from the Featured Image postbox thumbnail.

The title attribute is now replaced with plain text in a `howto` paragraph, as done for example in the Tags postbox.
Adds an `aria-describedby` attribute on the thumbnail link to target the description.
Also, fixes the focus style on the featured image thumbnail.

Fixes #35076.
Built from https://develop.svn.wordpress.org/trunk@36000


git-svn-id: https://core.svn.wordpress.org/trunk@35965 1a063a9b-81f0-0310-95a4-ce76da25c4cd