Create Light Client Update SSZ Responses based on Attested Header Slot and not Signature Slot

[Inspector-Butters]

The Fork Digest context in SSZ response for the /light_client/update endpoint is using signature_slot as it's source for calculation and not the update.attested_header.beacon.slot that the beacon API spec mentions.

This line in lighthouse.

Beacon API doc

[Inspector-Butters]

based on this comment by @etan-status the format of the SSZ response for this endpoint seems incorrect. CC. @eserilev

https://discord.com/channels/595666850260713488/595701319793377299/1351897494606315560