multi: use btcd's btcec/v2 and btcutil modules

This commit was previously split into the following parts to ease
review:
 - 2d746f68: replace imports
 - 4008f0fd: use ecdsa.Signature
 - 849e33d1: remove btcec.S256()
 - b8f6ebbd: use v2 library correctly
 - fa80bca9: bump go modules

lnwire/netaddress_test.go

@@ -5,7 +5,7 @@ import (
 	"net"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 )
 
 func TestNetAddressDisplay(t *testing.T) {
@@ -17,7 +17,7 @@ func TestNetAddressDisplay(t *testing.T) {
 		t.Fatalf("unable to decode str: %v", err)
 	}
 
-	pubKey, err := btcec.ParsePubKey(pubHex, btcec.S256())
+	pubKey, err := btcec.ParsePubKey(pubHex)
 	if err != nil {
 		t.Fatalf("unable to parse pubkey: %v", err)
 	}