Correct ChannelManager in_flight_monitor_updates serialization

In e8854f9 we changed the type of
`ChannelManager::in_flight_monitor_updates`, writing a legacy
version and reading the new version as a new TLV field. Sadly, we
spuriously marked the new TLV as `required`, breaking upgrade from
0.1.

Here we fix the oversight by simply marking it `option`al.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -12960,7 +12960,7 @@ where
 			(13, htlc_onion_fields, optional_vec),
 			(14, decode_update_add_htlcs_opt, option),
 			(15, self.inbound_payment_id_secret, required),
-			(17, in_flight_monitor_updates, required),
+			(17, in_flight_monitor_updates, option),
 		});
 
 		Ok(())
@@ -13503,7 +13503,7 @@ where
 			(13, claimable_htlc_onion_fields, optional_vec),
 			(14, decode_update_add_htlcs, option),
 			(15, inbound_payment_id_secret, option),
-			(17, in_flight_monitor_updates, required),
+			(17, in_flight_monitor_updates, option),
 		});
 		let mut decode_update_add_htlcs = decode_update_add_htlcs.unwrap_or_else(|| new_hash_map());
 		if fake_scid_rand_bytes.is_none() {