DropdownMenuV2: add GroupLabel subcomponent

[ciampo]

What?

Part of #50459

Add GroupLabel sub-component to DropdownMenuV2

Why?

As discussed in https://github.com/WordPress/gutenberg/pull/62880#discussion_r1689884904, there is a need for group labels in DropdownMenuV2.

How?

• Add a DropdownMenuV2.GroupLabel component based on Ariakit.MenuGroupLabel + <Text variant="muted" />
• Swap custom implementation in Block Bindings UI with the first party group label

Testing Instructions

Check Storybook examples

Testing Instructions for Keyboard

Screenshots or screencast

Click to see screenshots: