Update to execution-apis v1.0.0-alpha.6

[paulhauner]

Description

There is a new version of the execution API 🎉 https://github.com/ethereum/execution-apis/releases/tag/v1.0.0-alpha.6

We need to ensure Lighthouse is updated to match the new JSON structures and semantics.

It would be sensible to coordinate this with other EL and CL teams. If you're not sure how to go about this, reach out to me :)

[ethDreamer]

I might as well do this since I did the last one :)

[paulhauner]

FYI over in #2837 I made some changes to ensure we thread the latest_valid_hash back up to the BeaconChain, we'll have to make sure we do the same for the new execute payload response variants.

[paulhauner]

Resolved in #3018