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

lnrpc/invoicesrpc/utils.go

@@ -4,7 +4,7 @@ import (
 	"encoding/hex"
 	"fmt"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnrpc"
@@ -275,7 +275,7 @@ func CreateZpay32HopHints(routeHints []*lnrpc.RouteHint) ([][]zpay32.HopHint, er
 			if err != nil {
 				return nil, err
 			}
-			p, err := btcec.ParsePubKey(pubKeyBytes, btcec.S256())
+			p, err := btcec.ParsePubKey(pubKeyBytes)
 			if err != nil {
 				return nil, err
 			}