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

peer/brontide_test.go

@@ -6,11 +6,11 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/contractcourt"
@@ -1047,7 +1047,7 @@ func TestPeerCustomMessage(t *testing.T) {
 	dbAlice, err := channeldb.Open(alicePath)
 	require.NoError(t, err)
 
-	aliceKey, err := btcec.NewPrivateKey(btcec.S256())
+	aliceKey, err := btcec.NewPrivateKey()
 	require.NoError(t, err)
 
 	writeBufferPool := pool.NewWriteBuffer(