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

input/signdescriptor_test.go

@@ -5,7 +5,7 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -96,7 +96,7 @@ func TestSignDescriptorSerialization(t *testing.T) {
 	for i := 0; i < len(signDescriptors); i++ {
 		// Parse pubkeys for each sign descriptor.
 		sd := &signDescriptors[i]
-		pubkey, err := btcec.ParsePubKey(keys[i], btcec.S256())
+		pubkey, err := btcec.ParsePubKey(keys[i])
 		if err != nil {
 			t.Fatalf("unable to parse pubkey: %v", err)
 		}