Remove backward sync functionality from engine_newPayload.

[siladu]

Only trigger backward sync on engine_forkchoiceUpdated.

Fixes #5411

This is a significant improvement for at least besu-nimbus pairs, off the back off #5411 (comment)
and has been tested successfully with Teku and Nimbus, see POC #5666

Main thing to work out is if we should return SYNCING, ACCEPTED or something else in this case.

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Spec allows it:

https://github.com/ethereum/execution-apis/blob/main/src/engine/paris.md#specification
(newPayload)

Client software MAY initiate a sync process if requisite data for payload validation is missing. Sync process is specified in the Sync section.

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Geth impl: https://github.com/ethereum/go-ethereum/blob/59f7b289c329b5a56fa6f4e9acee64e504c4cc0d/eth/catalyst/api.go#L476-L485

Note, geth also stash away these newPayloads for later use. That would be a further optimism but is not critical for this issue.

[non-fungible-nelson]

@siladu - can we status this, #5622, and #5411?

Do we think #5699 solves these?

[non-fungible-nelson]

bump for status