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

discovery/reliable_sender_test.go

@@ -5,7 +5,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/lnpeer"
 	"github.com/lightningnetwork/lnd/lntest/wait"
@@ -20,7 +20,7 @@ func newTestReliableSender(t *testing.T) *reliableSender {
 	cfg := &reliableSenderCfg{
 		NotifyWhenOnline: func(pubKey [33]byte,
 			peerChan chan<- lnpeer.Peer) {
-			pk, err := btcec.ParsePubKey(pubKey[:], btcec.S256())
+			pk, err := btcec.ParsePubKey(pubKey[:])
 			if err != nil {
 				t.Fatalf("unable to parse pubkey: %v", err)
 			}