Change the mandatory checkpoint to Canopy

[teor2345]

Is your feature request related to a problem? Please describe.

Zebra should change its mandatory checkpoint to the Canopy network upgrade, to reduce the work we need to do for NU5 activation.

Checkpointing on Canopy lets us skip the following validation:

• transfers into the Sprout value pool (rejected at Canopy activation)
• Founders Reward (finished right before Canopy activation)
• validation of the PreSapling, Sapling, and ChainHistoryActivation (Heartwood activation) RootHash variants
• the pre-Blossom block target spacing (finished right before Blossom activation)

Describe the solution you'd like

• Update the list of Zebra checkpoints for mainnet and testnet
• Change the Sapling mandatory checkpoint to Canopy

Describe alternatives you've considered

Write a whole lot of validation code that we'll never really use.