DropdownMenuV2: update animation

[ciampo]

What?

Part of #50459

Updates DropdownMenuV2 animation following the latest specs (shared by @nick-a8c)

Why?

For a better sense of polish, and alignment with the rest of the animations in Gutenberg.

How?

The new animation specs require the popover wrapper and the popover contents to scale at different rates. That required a small refactor to include an extra wrapper element, to account for the two separate animations (proof of concept: https://codepen.io/ciampo/pen/mdZjPWg)

The scale animation does not happen when the popover opens to the side of its trigger — in that case, the popover only fades in.

Testing Instructions

Check DropdownMenuV2 examples in Storybook and its usages across the editor, make sure the animation looks good and follows the specs, without impactive the usability and the semantics of the component.-->

Screenshots or screencast

https://github.com/user-attachments/assets/33a8d47d-a8f8-402a-a1b6-6b0c21098add