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

lnwire/lnwire.go

@@ -9,10 +9,10 @@ import (
 	"math"
 	"net"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/go-errors/errors"
 	"github.com/lightningnetwork/lnd/tor"
 )
@@ -541,7 +541,7 @@ func ReadElement(r io.Reader, element interface{}) error {
 			return err
 		}
 
-		pubKey, err := btcec.ParsePubKey(b[:], btcec.S256())
+		pubKey, err := btcec.ParsePubKey(b[:])
 		if err != nil {
 			return err
 		}