Replace div with button so media attachment modal can be closed with keyboard

[joedolson]

The modal for attaching a media item to a post uses a <div> for a close element. Switches to a button.

Probably should also add some focus management into media.js so that focus lands in an appropriate place; not handled in this patch.

[joedolson]

Switch from div to button

[afercia]

Yep this modal dialog needs a complete accessibility treatment, as done for example for the "advanced" editor link modal. In the meanwhile this is good to go. Changed just one thing: wouldn't recommend to use the CSS class button-link here because it's going to be modified and have an underline and other new things in #35126.

[afercia]

In 37231:

Accessibility: change the close "X" in the Media "Attach to existing content" modal in a button.

Props joedolson.

Fixes #36554.