Fix caplin RPC attestation rewards being off by one epoch (#12073)

Checked against beaconcha.in, against a synced prysm, and against
ethereum-beacon-api.publicnode.com.

Catlin is consistently off-by-one (one has to ask for the next numbered
epoch to get calculation for the one that one wanted).

@Giulio2002

cl/beacon/handler/attestation_rewards.go

@@ -67,6 +67,10 @@ func (a *ApiHandler) PostEthV1BeaconRewardsAttestations(w http.ResponseWriter, r
 	defer tx.Rollback()
 
 	epoch, err := beaconhttp.EpochFromRequest(r)
+
+	// attestation rewards are always delayed by 1 extra epoch compared to other rewards
+	epoch += 1
+
 	if err != nil {
 		return nil, beaconhttp.NewEndpointError(http.StatusBadRequest, err)
 	}

cl/beacon/handler/test_data/attestations_2.json

@@ -1 +1 @@
-{"data":{"ideal_rewards":[{"effective_balance":"32000000000","head":"0","target":"290680","source":"0","inclusion_delay":"0","inactivity":"0"},{"effective_balance":"32000000000","head":"0","target":"290680","source":"0","inclusion_delay":"0","inactivity":"0"}],"total_rewards":[{"validator_index":"1","head":"0","target":"290680","source":"-156520","inclusion_delay":"0","inactivity":"0"},{"validator_index":"4","head":"0","target":"290680","source":"-156520","inclusion_delay":"0","inactivity":"0"}]},"execution_optimistic":false,"finalized":true}
+{"data":{"ideal_rewards":[{"effective_balance":"32000000000","head":"0","target":"9083","source":"0","inclusion_delay":"0","inactivity":"0"},{"effective_balance":"32000000000","head":"0","target":"9083","source":"0","inclusion_delay":"0","inactivity":"0"}],"total_rewards":[{"validator_index":"1","head":"0","target":"-290680","source":"-156520","inclusion_delay":"0","inactivity":"0"},{"validator_index":"4","head":"0","target":"-290680","source":"-156520","inclusion_delay":"0","inactivity":"0"}]},"execution_optimistic":false,"finalized":true}