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/addinvoice.go

@@ -9,10 +9,10 @@ import (
 	"math"
 	"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/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lntypes"

lnrpc/invoicesrpc/addinvoice_test.go

@@ -3,10 +3,9 @@ package invoicesrpc
 import (
 	"encoding/hex"
 	"errors"
-	"math/big"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -57,11 +56,12 @@ func TestSelectHopHints(t *testing.T) {
 			"598ec453728e0ffe0ae2f5e174243cf58f2" +
 				"a3f2c83d2457b43036db568b11093",
 		)
-		pubkey = &btcec.PublicKey{
-			X:     big.NewInt(4),
-			Y:     new(big.Int).SetBytes(pubkeyBytes),
-			Curve: btcec.S256(),
-		}
+		pubKeyY = new(btcec.FieldVal)
+		_       = pubKeyY.SetByteSlice(pubkeyBytes)
+		pubkey  = btcec.NewPublicKey(
+			new(btcec.FieldVal).SetInt(4),
+			pubKeyY,
+		)
 		compressed = pubkey.SerializeCompressed()
 
 		publicChannel = &HopHintInfo{

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
 			}