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/wtclient/backup_task_internal_test.go

@@ -7,11 +7,11 @@ import (
 	"reflect"
 	"testing"
 
-	"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/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/input"
@@ -103,13 +103,13 @@ func genTaskTest(
 
 	// Parse the key pairs for all keys used in the test.
 	revSK, revPK := btcec.PrivKeyFromBytes(
-		btcec.S256(), revPrivBytes,
+		revPrivBytes,
 	)
 	_, toLocalPK := btcec.PrivKeyFromBytes(
-		btcec.S256(), toLocalPrivBytes,
+		toLocalPrivBytes,
 	)
 	toRemoteSK, toRemotePK := btcec.PrivKeyFromBytes(
-		btcec.S256(), toRemotePrivBytes,
+		toRemotePrivBytes,
 	)
 
 	// Create the signer, and add the revocation and to-remote privkeys.