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

routing/payment_lifecycle.go

@@ -5,7 +5,7 @@ import (
 	"sync"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/davecgh/go-spew/spew"
 	sphinx "github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/channeldb"
@@ -883,9 +883,7 @@ func (p *shardHandler) handleFailureMessage(rt *route.Route,
 	// always succeed, otherwise there is something wrong in our
 	// implementation. Therefore return an error.
 	errVertex := rt.Hops[errorSourceIdx-1].PubKeyBytes
-	errSource, err := btcec.ParsePubKey(
-		errVertex[:], btcec.S256(),
-	)
+	errSource, err := btcec.ParsePubKey(errVertex[:])
 	if err != nil {
 		log.Errorf("Cannot parse pubkey: idx=%v, pubkey=%v",
 			errorSourceIdx, errVertex)