[6.x] Give modals a dialog role and accessible name

[lazerg]

This pull request fixes an issue where the Control Panel's modals are announced as plain groups of text rather than dialogs. Every confirmation modal inherits from ui/Modal, so deleting an entry, removing a nav page or dismissing the licensing alert gives a screen reader a Cancel/Delete choice with no indication that a dialog opened, and no name for it.

This was happening because the modal content is a bare <div data-ui-modal-content> with no role, no aria-modal and no aria-labelledby. The command palette avoids this because it uses reka-ui's DialogContent instead of this component.

This PR fixes it by adding role="dialog" and aria-modal="true" to the content element, and pointing aria-labelledby at the title the modal already renders. The title gets an id from useId(), shared with ModalTitle through a provide so both ways of setting a title work. aria-labelledby is left off when there's no title at all, and the attributes come before the fallthrough attrs so a consumer can still pass role="alertdialog".

Focus is already trapped by the FocusScope, so I left the inert suggestion out of this one.

Fixes #15410