Confirmation rule prerequisite

[twoeths]

Problem description

We need some forkchoice changes for the safe execution of the confirmation rule, here's the main PR ethereum/consensus-specs#3339

Solution description

specified in ethereum/consensus-specs#3431

Additional context

No response

[philknows]

Consensus spec PR will get merged shortly with spec release around Monday. We need to get this out for mainnet fork of Deneb @tuyennhv. Mainnet fork date bit of a moving target depending on testnet performance.