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

lnwallet/transactions_test.go

@@ -15,11 +15,11 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -511,15 +511,15 @@ func testSpendValidation(t *testing.T, tweakless bool) {
 	// Each side currently has 1 BTC within the channel, with a total
 	// channel capacity of 2BTC.
 	aliceKeyPriv, aliceKeyPub := btcec.PrivKeyFromBytes(
-		btcec.S256(), testWalletPrivKey,
+		testWalletPrivKey,
 	)
 	bobKeyPriv, bobKeyPub := btcec.PrivKeyFromBytes(
-		btcec.S256(), bobsPrivKey,
+		bobsPrivKey,
 	)
 
 	revocationPreimage := testHdSeed.CloneBytes()
 	commitSecret, commitPoint := btcec.PrivKeyFromBytes(
-		btcec.S256(), revocationPreimage,
+		revocationPreimage,
 	)
 	revokePubKey := input.DeriveRevocationPubkey(bobKeyPub, commitPoint)
 
@@ -783,7 +783,7 @@ func createTestChannelsForVectors(tc *testContext, chanType channeldb.ChannelTyp
 		&remoteDummy1, &remoteDummy2, &localDummy1, &localDummy2,
 	}
 	for _, keyRef := range generateKeys {
-		privkey, err := btcec.NewPrivateKey(btcec.S256())
+		privkey, err := btcec.NewPrivateKey()
 		require.NoError(t, err)
 		*keyRef = privkey
 	}