Make sure the mandatory checkpoint includes the Canopy activation block

[mpguerra]

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

In general, the chain history for each block commits to all the blocks between:

• the previous network upgrade activation + 1 and
• the previous block

So the Canopy activation block commits to all the blocks between Heartwood activation + 1 and Canopy activation - 1.

If we change the mandatory checkpoint to Canopy activation + 1, we will be able to skip calculating all the heartwood chain history.

The ChainVerifier checkpoints all blocks up to and including the Canopy activation maximum height. Zebra chooses the next checkpoint at or after the mandatory height.

So the actual checkpoint does not need to change on either network:

Network	Canopy	Zebra Checkpoint
Mainnet	1028500	1028849
Testnet	1046400	1046400

But we do need to update the finalized state assertion, the documentation, and the checkpoint tests.

Describe the solution you'd like

• assert that the canopy activation block is a finalized checkpoint block
• clarify the checkpoint test docs - they're already checking that canopy activation is a checkpoint block
• update Zebra's checkpoint docs to explicitly include Canopy activation
• if the mandatory checkpoint appears in multiple places, turn it into a named constant (optional)

The hard-coded checkpoint lists extend a long way past Canopy, so they don't need to be updated.

Additional context

See #2091 for more details

[teor2345]

Zebra chooses the next checkpoint at or after the mandatory Canopy activation height. And the ChainVerifier checkpoints all blocks up to and including the checkpoint.

So we don't need to change the actual checkpoint - the first non-finalized block is Canopy activation + 1 block.

We should fix the state assertion, the test, the checkpoint documentation though.