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

netann/channel_update_test.go

@@ -5,7 +5,8 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
 	"github.com/lightningnetwork/lnd/keychain"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -18,7 +19,7 @@ type mockSigner struct {
 }
 
 func (m *mockSigner) SignMessage(_ keychain.KeyLocator,
-	_ []byte, _ bool) (*btcec.Signature, error) {
+	_ []byte, _ bool) (*ecdsa.Signature, error) {
 
 	if m.err != nil {
 		return nil, m.err
@@ -30,7 +31,7 @@ func (m *mockSigner) SignMessage(_ keychain.KeyLocator,
 var _ lnwallet.MessageSigner = (*mockSigner)(nil)
 
 var (
-	privKey, _    = btcec.NewPrivateKey(btcec.S256())
+	privKey, _    = btcec.NewPrivateKey()
 	privKeySigner = keychain.NewPrivKeyMessageSigner(privKey, testKeyLoc)
 
 	pubKey = privKey.PubKey()