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

netann/chan_status_manager_test.go

@@ -11,7 +11,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -80,7 +80,7 @@ func createEdgePolicies(t *testing.T, channel *channeldb.OpenChannel,
 	}
 
 	// Generate and set pubkey2 for THEIR pubkey.
-	privKey2, err := btcec.NewPrivateKey(btcec.S256())
+	privKey2, err := btcec.NewPrivateKey()
 	if err != nil {
 		t.Fatalf("unable to generate key pair: %v", err)
 	}
@@ -310,7 +310,7 @@ func newManagerCfg(t *testing.T, numChannels int,
 
 	t.Helper()
 
-	privKey, err := btcec.NewPrivateKey(btcec.S256())
+	privKey, err := btcec.NewPrivateKey()
 	if err != nil {
 		t.Fatalf("unable to generate key pair: %v", err)
 	}