Add checkpoints for Axion upgrade

Summary:
See title. Will wait to land until after these blocks are buried sufficiently deep.

Left off BCHN's testnet chain for now, as it's not clear if this will be stable or not.

Test Plan: Run IBD on CI

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8422