multi: Upgrade to ldk v20

We update to ldk v20 for the latest blinded route updates, namely being
able to create a blinded route that's more than one hop. On top of that we use
a custom versoin with a small change, which we need in order to send an onion
message along a specified path for LNDK's integration tests:
lightningdevkit/rust-lightning#2868

Author: orbitalturtle

tests/integration_tests.rs

@@ -63,6 +63,7 @@ async fn test_lndk_forwards_onion_message() {
     // Notice that ldk1 and ldk2 are not connected directly to each other.
     let (pubkey, addr) = ldk1.get_node_info();
     let (pubkey_2, addr_2) = ldk2.get_node_info();
+
     lnd.connect_to_peer(pubkey, addr).await;
     lnd.connect_to_peer(pubkey_2, addr_2).await;
     let lnd_info = lnd.get_info().await;