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/sigpool.go

@@ -4,7 +4,8 @@ import (
 	"fmt"
 	"sync"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwire"
@@ -35,7 +36,7 @@ type VerifyJob struct {
 
 	// Sig is the raw signature generated using the above public key.  This
 	// is the signature to be verified.
-	Sig *btcec.Signature
+	Sig *ecdsa.Signature
 
 	// SigHash is a function closure generates the sighashes that the
 	// passed signature is known to have signed.