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

watchtower/blob/justice_kit.go

@@ -8,7 +8,7 @@ import (
 	"fmt"
 	"io"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -153,14 +153,14 @@ type JusticeKit struct {
 // commitment to-local output.
 func (b *JusticeKit) CommitToLocalWitnessScript() ([]byte, error) {
 	revocationPubKey, err := btcec.ParsePubKey(
-		b.RevocationPubKey[:], btcec.S256(),
+		b.RevocationPubKey[:],
 	)
 	if err != nil {
 		return nil, err
 	}
 
 	localDelayedPubKey, err := btcec.ParsePubKey(
-		b.LocalDelayPubKey[:], btcec.S256(),
+		b.LocalDelayPubKey[:],
 	)
 	if err != nil {
 		return nil, err
@@ -206,9 +206,7 @@ func (b *JusticeKit) CommitToRemoteWitnessScript() ([]byte, error) {
 	// If this is a blob for an anchor channel, we'll return the p2wsh
 	// output containing a CSV delay of 1.
 	if b.BlobType.IsAnchorChannel() {
-		pk, err := btcec.ParsePubKey(
-			b.CommitToRemotePubKey[:], btcec.S256(),
-		)
+		pk, err := btcec.ParsePubKey(b.CommitToRemotePubKey[:])
 		if err != nil {
 			return nil, err
 		}