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
- [x] 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