Fix local testnets starting from Electra at genesis

[michaelsproul]

At the moment we get an SSZ decoding error trying to start a Kurtosis testnet with ELECTRA_FORK_EPOCH: 0.

Jan 21 01:01:17.667 CRIT Failed to start beacon node reason: Built-in genesis state SSZ bytes are invalid: OffsetOutOfBounds(522733568)

[michaelsproul]

This seems to be an issue upstream in ethereum-genesis-generator, possibly eth2-testnet-genesis. The state generated with ELECTRA_FORK_EPOCH: 0 is actually a Deneb state, as confirmed with:

lcli pretty-ssz BeaconStateDeneb genesis.ssz

[michaelsproul]

Have asked maintainers, but the only PR I can find related to Electra is:

• add support for electra/kaustinen genesis protolambda/eth2-testnet-genesis#17