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

chainntnfs/test_utils.go

@@ -14,14 +14,14 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/btcjson"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/integration/rpctest"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/btcsuite/btcwallet/chain"
 	"github.com/btcsuite/btcwallet/walletdb"
 	"github.com/lightninglabs/neutrino"
@@ -42,7 +42,7 @@ var (
 // randPubKeyHashScript generates a P2PKH script that pays to the public key of
 // a randomly-generated private key.
 func randPubKeyHashScript() ([]byte, *btcec.PrivateKey, error) {
-	privKey, err := btcec.NewPrivateKey(btcec.S256())
+	privKey, err := btcec.NewPrivateKey()
 	if err != nil {
 		return nil, nil, err
 	}