Review when to send FCU in gloas

> this is not needed for alpha.5 we relay on shouldExtendPayload() instead see #9209

https://github.com/ChainSafe/lodestar/pull/9164#discussion_r3066656116 this should still be relevant, we can't always send the FCU at 8 seconds into the slot anymore since the payload may arrive later, my gut feeling is that 10 seconds in the right number or as soon as we have enough PTC votes, but nothing that's critical for the devnet

_Originally posted by @nflaig in https://github.com/ChainSafe/lodestar/issues/9164#issuecomment-4287314252_