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

channeldb/migration24/migration_test.go

@@ -9,10 +9,10 @@ import (
 	"math/rand"
 	"testing"
 
-	"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"
 	lnwire "github.com/lightningnetwork/lnd/channeldb/migration/lnwire21"
 	mig "github.com/lightningnetwork/lnd/channeldb/migration_01_to_11"
 	"github.com/lightningnetwork/lnd/channeldb/migtest"
@@ -62,7 +62,7 @@ var (
 		},
 		LockTime: 5,
 	}
-	_, pubKey = btcec.PrivKeyFromBytes(btcec.S256(), key[:])
+	_, pubKey = btcec.PrivKeyFromBytes(key[:])
 )
 
 // TestMigrateFwdPkgCleanup asserts that the migration will delete all the