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

lnrpc/walletrpc/walletkit_server.go

@@ -14,13 +14,13 @@ import (
 	"path/filepath"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"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/txscript"
 	"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/wtxmgr"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
@@ -1526,7 +1526,7 @@ func (w *WalletKit) ImportAccount(ctx context.Context,
 func (w *WalletKit) ImportPublicKey(ctx context.Context,
 	req *ImportPublicKeyRequest) (*ImportPublicKeyResponse, error) {
 
-	pubKey, err := btcec.ParsePubKey(req.PublicKey, btcec.S256())
+	pubKey, err := btcec.ParsePubKey(req.PublicKey)
 	if err != nil {
 		return nil, err
 	}