Enable SNAP history expiry sync by default

[siladu]

The idea is to use the existing --Xsnapsync-synchronizer-pre-merge-headers-only-enabled flag for sepolia initially and then enable it by default for the mainnet release.

It would be prudent to leave the flag in place so we can disable this functionality to allow for a full block history SNAP (the data may still available over devp2p).

Consider renaming the flag to --snapsync-synchronizer-pre-checkpoint-headers-only-enabled since it can work with private network checkpoints as well.

Pruning requires a separate flag, --Xhistory-expiry-prune (and an optional subcommand) but that shouldn't be enabled by default.
Once we're happy with Sepolia, we should promote to --history-expiry-prune though.

We should enable eth/69 at the same time.

Consider appropriate defaults for private networks:

• Since private networks generally don't use a checkpoint, enabling this by default shouldn't impact them: no checkpoint === checkpoint = block 0.

[siladu]

Blocked until mainnet date is agreed, and this should be release on or after that date

[siladu]

No coordinated mainnet date required, but will do some more testing on our canaries for a week or two before we play this ticket.