Use Status V2 `earliest_available_slot` for syncing

[jimmygchen]

Spec: ethereum/consensus-specs#4374

The above spec change has been partially implemented (#7590) and Lighthouse now serves and exchanges Status V2.

The remaining work:

1. Utilise the new earliest_available_slot info for peer selection during sync
2. Update the earliest_available_slot when (covered in Sync peer attribution #7733 and Implement validator custody backfill when CGC increases #7603)
   • CGC increases (set to the slot when the effective CGC becomes effective and all advertises columns will be available)
   • Backfill (not implemented)

[jimmygchen]

Partially implemented in #7733. This still need to be done for backfill.

[jimmygchen]

Completed in #7733 and #7762