Even after we remove the deprecated API, we will want to maintain a minimal
API for internal use, in order to avoid importing the external
tailscale.com/client/tailscale/v2 package. This shim exposes only the necessary
parts of the deprecated API for internal use, which gains us the following:
1. It removes deprecation warnings for internal use of the API.
2. It gives us an inventory of which parts we will want to keep for internal use.
Updates tailscale/corp#22748
Signed-off-by: Percy Wegmann <percy@tailscale.com>