chore: use portal by default in avatar menu

[oliverchanyw]

Problem

Bright uses the AvatarMenu in the app NavBar, which is a fairly typical use case. It is as sticky bar, which creates its own stacking context. Unrelated ancestor props like maxHeight or overflow then cause issues for the AvatarMenu.

Solution

There's rarely a reason not to use Menu with Portal. This changes AvatarMenu to do that, without changing the component signature.

Alternative considered

Accept a new custom prop like withPortal. Rejected because it is custom, and it feels acceptable to deny users the option to not use portal.

[oliverchanyw]

hm why is the portal flaking the menu :/