Fix Withdrawal serialisation and check address change fork (#3789)

* Disallow address changes before Capella

* Quote u64s in Withdrawal serialisation

beacon_node/beacon_chain/src/errors.rs

@@ -206,6 +206,7 @@ pub enum BeaconChainError {
     MissingPersistedForkChoice,
     CommitteePromiseFailed(oneshot_broadcast::Error),
     MaxCommitteePromises(usize),
+    BlsToExecutionChangeBadFork(ForkName),
 }
 
 easy_from_to!(SlotProcessingError, BeaconChainError);