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/routerrpc/router_backend.go

@@ -9,10 +9,10 @@ import (
 	math "math"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/feature"
 	"github.com/lightningnetwork/lnd/htlcswitch"
@@ -927,7 +927,7 @@ func unmarshallHopHint(rpcHint *lnrpc.HopHint) (zpay32.HopHint, error) {
 		return zpay32.HopHint{}, err
 	}
 
-	pubkey, err := btcec.ParsePubKey(pubBytes, btcec.S256())
+	pubkey, err := btcec.ParsePubKey(pubBytes)
 	if err != nil {
 		return zpay32.HopHint{}, err
 	}

lnrpc/routerrpc/router_backend_test.go

@@ -6,7 +6,7 @@ import (
 	"encoding/hex"
 	"testing"
 
-	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/lnwire"

lnrpc/routerrpc/router_server.go

@@ -10,8 +10,8 @@ import (
 	"sync/atomic"
 	"time"
 
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lnrpc"

lnrpc/routerrpc/routing_config.go

@@ -3,7 +3,7 @@ package routerrpc
 import (
 	"time"
 
-	"github.com/btcsuite/btcutil"
+	"github.com/btcsuite/btcd/btcutil"
 )
 
 // RoutingConfig contains the configurable parameters that control routing.