[tooltip] Update to use native popover for the overlay element

[web-padawan]

Motivation

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

Proposed solution

1. Keep vaadin-tooltip-overlay in shadow DOM and use exportparts for ::part() styling
2. Pass text via the Lit template instead of using a renderer and requestContentUpdate()
3. OR make the slotted element used sr-label with role="tooltip" an actual visible child

IMO using slotted element makes more sense, we should then change slot="sr-label" to e.g. slot="overlay".

Note

This will break some workarounds mentioned internally for rendering HTML content that rely on patching _renderer.
IMO it's fine and we should encourage developers to instead use vaadin-popover where possible.