Fix merge rpc length limits (#3133)

## Issue Addressed

N/A

## Proposed Changes

Fix the upper bound for blocks by root responses to be equal to the max merge block size instead of altair.
Further make the rpc response limits fork aware.

beacon_node/lighthouse_network/src/rpc/handler.rs

@@ -477,7 +477,7 @@ where
                 ProtocolError::InvalidMessage | ProtocolError::TooManyProtocols => {
                     // Peer is sending invalid data during the negotiation phase, not
                     // participating in the protocol
-                    RPCError::InvalidData
+                    RPCError::InvalidData("Invalid message during negotiation".to_string())
                 }
             },
         };