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

htlcswitch/mock.go

@@ -14,9 +14,9 @@ import (
 	"testing"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/go-errors/errors"
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/chainntnfs"
@@ -610,7 +610,7 @@ func (s *mockServer) PubKey() [33]byte {
 }
 
 func (s *mockServer) IdentityKey() *btcec.PublicKey {
-	pubkey, _ := btcec.ParsePubKey(s.id[:], btcec.S256())
+	pubkey, _ := btcec.ParsePubKey(s.id[:])
 	return pubkey
 }