[crud] Move confirm-dialog to the slot to make its overlay stylable globally

[web-padawan]

Motivation

With native popover, we'll no longer teleport vaadin-confirm-dialog-overlay to the body but instead keep it in the shadow DOM of the vaadin-confirm-dialog. We should make sure it can be styled with light DOM CSS.

Proposed solution

Move confirm-dialog used in vaadin-crud the light DOM e.g. slot="confirm-dialog".