isValidAttestationData returns invalid result for earlier epochs

[nazarhussain]

Describe the bug

When called isValidAttestationData for the an attestation which is targeting last slot of the second epoch it results to zero.

It's because of the following condition always evaluates to -1 for which we don't have a block header.

lodestar/packages/beacon-node/src/chain/opPools/aggregatedAttestationPool.ts

Line 435 in 0886d75

const pivotSlot = computeStartSlotAtEpoch(targetEpoch - 1) - 1;

Expected behavior

Instead of a hardcoded condition for first 2 epochs, we should simply rely if the pivotSlot is not the negative. That will suffice the few first epochs condition as well.

lodestar/packages/beacon-node/src/chain/opPools/aggregatedAttestationPool.ts

Lines 430 to 432 in 0886d75

	if (stateEpoch < 2) {
	return true;
	}

Desktop (please complete the following information):

• OS: Mac
• Branch: [unstable]