* Test if mandatory checkpoint is after grace period
Check that the mandatory checkpoint height is after the ZIP-212 grace
period for both mainnet and testnet, because Zebra is currently unable
to fully validate the blocks during that grace period.
* Update mandatory checkpoint height
Require it to be after the ZIP-212 grace period so that the blocks
during the grace period are validated by the checkpoints because Zebra
can't currently fully validate blocks during that period.