Prevent attestation to future blocks from early attester cache (#3183)

## Issue Addressed

N/A

## Proposed Changes

Prevents the early attester cache from producing attestations to future blocks. This bug could result in a missed head vote if the BN was requested to produce an attestation for an earlier slot than the head block during the (usually) short window of time between verifying a block and setting it as the head.

This bug was noticed in an [Antithesis](https://andreagrieser.com/) test and diagnosed by @realbigsean. 

## Additional Info

NA

beacon_node/beacon_chain/src/early_attester_cache.rs

@@ -104,6 +104,10 @@ impl<E: EthSpec> EarlyAttesterCache<E> {
             return Ok(None);
         }
 
+        if request_slot < item.block.slot() {
+            return Ok(None);
+        }
+
         let committee_count = item
             .committee_lengths
             .get_committee_count_per_slot::<E>(spec)?;

beacon_node/beacon_chain/tests/attestation_production.rs

@@ -148,3 +148,58 @@ fn produces_attestations() {
         }
     }
 }
+
+/// Ensures that the early attester cache wont create an attestation to a block in a later slot than
+/// the one requested.
+#[test]
+fn early_attester_cache_old_request() {
+    let harness = BeaconChainHarness::builder(MainnetEthSpec)
+        .default_spec()
+        .keypairs(KEYPAIRS[..].to_vec())
+        .fresh_ephemeral_store()
+        .mock_execution_layer()
+        .build();
+
+    harness.advance_slot();
+
+    harness.extend_chain(
+        2,
+        BlockStrategy::OnCanonicalHead,
+        AttestationStrategy::AllValidators,
+    );
+
+    let head = harness.chain.head().unwrap();
+    assert_eq!(head.beacon_block.slot(), 2);
+    let head_proto_block = harness
+        .chain
+        .fork_choice
+        .read()
+        .get_block(&head.beacon_block_root)
+        .unwrap();
+
+    harness
+        .chain
+        .early_attester_cache
+        .add_head_block(
+            head.beacon_block_root,
+            head.beacon_block.clone(),
+            head_proto_block,
+            &head.beacon_state,
+            &harness.chain.spec,
+        )
+        .unwrap();
+
+    let attest_slot = head.beacon_block.slot() - 1;
+    let attestation = harness
+        .chain
+        .produce_unaggregated_attestation(attest_slot, 0)
+        .unwrap();
+
+    assert_eq!(attestation.data.slot, attest_slot);
+    let attested_block = harness
+        .chain
+        .get_blinded_block(&attestation.data.beacon_block_root)
+        .unwrap()
+        .unwrap();
+    assert_eq!(attested_block.slot(), attest_slot);
+}