Deprecate CHECKPOINT for public networks

[siladu]

With these history expiry PRs:
#8584
#8582
#8634

The checkpoint has moved from the deposit contract deployment block to the first PoS block.

For mainnet once history expiry is live, SNAP by default will use this checkpoint to only save headers before it, and full blocks after it.
This differs from the current behaviour of CHECKPOINT which will delete the full block.

CHECKPOINT existed to speed up sync, however by default SNAP will now be faster so there isn't a need for CHECKPOINT on public Ethereum once history expiry is live.

Some users will still be using CHECKPOINT so we should migrate them to SNAP.
Can add a warning log that fires for mainnet at least, and deprecate in the code/docs.

We should also consider what to do for private networks.

One potential way to retain the same functionality as CHECKPOINT (deleting the full block) is to add a new SNAP config option to switch on saving headers or not.

[jflo]

@matthew1001 are private networks using CHECKPOINT sync? It seems like they could benefit from it if they have long blockchains they don't care about syncing.