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

chanacceptor/acceptor_test.go

@@ -2,13 +2,12 @@ package chanacceptor
 
 import (
 	"errors"
-	"math/big"
 	"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/btcutil"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnwallet/chancloser"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -126,10 +125,10 @@ func (c *channelAcceptorCtx) stop() {
 // request in a goroutine and then asserts that we get the outcome we expect.
 func (c *channelAcceptorCtx) queryAndAssert(queries map[*lnwire.OpenChannel]*ChannelAcceptResponse) {
 	var (
-		node = &btcec.PublicKey{
-			X: big.NewInt(1),
-			Y: big.NewInt(1),
-		}
+		node = btcec.NewPublicKey(
+			new(btcec.FieldVal).SetInt(1),
+			new(btcec.FieldVal).SetInt(1),
+		)
 
 		responses = make(chan struct{})
 	)