The upstream module has seen significant work making
the v1 emulation layer a high fidelity re-implementation
of v1 "encoding/json".
This addresses several upstream breaking changes:
* MarshalJSONV2 renamed as MarshalJSONTo
* UnmarshalJSONV2 renamed as UnmarshalJSONFrom
* Options argument removed from MarshalJSONV2
* Options argument removed from UnmarshalJSONV2
Updates tailscale/corp#791
Signed-off-by: Joe Tsai <joetsai@digital-static.net>