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

autopilot/prefattach_test.go

@@ -8,8 +8,8 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
-	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 )
 
@@ -252,9 +252,7 @@ func TestPrefAttachmentSelectGreedyAllocation(t *testing.T) {
 				t1.Fatalf("unable to create channel: %v", err)
 			}
 			peerPubBytes := edge1.Peer.PubKey()
-			peerPub, err := btcec.ParsePubKey(
-				peerPubBytes[:], btcec.S256(),
-			)
+			peerPub, err := btcec.ParsePubKey(peerPubBytes[:])
 			if err != nil {
 				t.Fatalf("unable to parse pubkey: %v", err)
 			}