Ignore attestations to finalized blocks (don't reject) (#3052)

## Issue Addressed

Addresses spec changes from v1.1.0:

- https://github.com/ethereum/consensus-specs/pull/2830
- https://github.com/ethereum/consensus-specs/pull/2846

## Proposed Changes

* Downgrade the REJECT for `HeadBlockFinalized` to an IGNORE. This applies to both unaggregated and aggregated attestations.

## Additional Info

I thought about also changing the penalty for `UnknownTargetRoot` but I don't think it's reachable in practice.

beacon_node/beacon_chain/src/attestation_verification.rs

@@ -961,7 +961,6 @@ impl<'a, T: BeaconChainTypes> VerifiedUnaggregatedAttestation<'a, T> {
 }
 
 /// Returns `Ok(())` if the `attestation.data.beacon_block_root` is known to this chain.
-/// You can use this `shuffling_id` to read from the shuffling cache.
 ///
 /// The block root may not be known for two reasons:
 ///