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/interface.go

@@ -6,12 +6,13 @@ import (
 	"sync"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/hdkeychain"
+	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/hdkeychain"
-	"github.com/btcsuite/btcutil/psbt"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/btcsuite/btcwallet/wallet/txauthor"
 	"github.com/btcsuite/btcwallet/wtxmgr"
@@ -468,7 +469,7 @@ type MessageSigner interface {
 	// be found, then an error will be returned. The actual digest signed is
 	// the single or double SHA-256 of the passed message.
 	SignMessage(keyLoc keychain.KeyLocator, msg []byte,
-		doubleHash bool) (*btcec.Signature, error)
+		doubleHash bool) (*ecdsa.Signature, error)
 }
 
 // WalletDriver represents a "driver" for a particular concrete