Introduce nv21 skeleton in Lotus

[rjan90]

The Filecoin nv21 upgrade, scope and timeline is being planned here.

In Lotus we need to add the skeleton for nv21 without any upgrade epochs, code / state changes / migrations. It should have no effect except on local devnets to unblock testing and tests.

Tasks

Beta Give feedback

1. Lotus actor version integration checklist: https://github.com/filecoin-project/lotus/blob/3636af4cd926a1f1fc2dadbb235bc01af047c75b/documentation/misc/actors_version_checklist.md#L8

Example for nv19:
Lotus: #10592

[jennijuju]

can this be closed now?

[rjan90]

Yeah, closed by #11176