[avatar-group] Update to use native popover for the overlay element

[web-padawan]

Motivation

The vaadin-avatar-group should set popover="manual" on the vaadin-avatar-group-overlay.

Proposed solution

1. Keep vaadin-avatar-group-overlay in shadow DOM and use exportparts for ::part() styling
2. Add slotted <vaadin-avatar-group-menu slot="overlay"> instead of using a renderer function