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

lncfg/address.go

@@ -9,7 +9,7 @@ import (
 	"strconv"
 	"strings"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/tor"
 )
@@ -294,7 +294,7 @@ func ParseLNAddressString(strAddress string, defaultPort string,
 
 	// Parse the pubkey bytes to verify that it corresponds to valid public
 	// key on the secp256k1 curve.
-	pubKey, err := btcec.ParsePubKey(pubKeyBytes, btcec.S256())
+	pubKey, err := btcec.ParsePubKey(pubKeyBytes)
 	if err != nil {
 		return nil, fmt.Errorf("invalid lightning address pubkey: %v", err)
 	}

lncfg/address_test.go

@@ -9,7 +9,7 @@ import (
 	"net"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/stretchr/testify/require"
 )
 
@@ -140,7 +140,7 @@ var (
 
 	pubKeyHex = hex.EncodeToString(pubKeyBytes)
 
-	pubKey, _ = btcec.ParsePubKey(pubKeyBytes, btcec.S256())
+	pubKey, _ = btcec.ParsePubKey(pubKeyBytes)
 )
 
 type lnAddressCase struct {