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/channel_test.go

@@ -12,10 +12,10 @@ import (
 	"testing/quick"
 
 	"github.com/btcsuite/btcd/blockchain"
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
@@ -4415,7 +4415,7 @@ func TestChanSyncFailure(t *testing.T) {
 	}
 	p := bobSyncMsg.LocalUnrevokedCommitPoint.SerializeCompressed()
 	p[4] ^= 0x01
-	modCommitPoint, err := btcec.ParsePubKey(p, btcec.S256())
+	modCommitPoint, err := btcec.ParsePubKey(p)
 	if err != nil {
 		t.Fatalf("unable to parse pubkey: %v", err)
 	}