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/migration21/current/current_codec.go

@@ -6,10 +6,10 @@ import (
 	"fmt"
 	"io"
 
-	"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"
 	"github.com/lightningnetwork/lnd/channeldb/migration21/common"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -317,7 +317,7 @@ func ReadElement(r io.Reader, element interface{}) error {
 			return err
 		}
 
-		priv, _ := btcec.PrivKeyFromBytes(btcec.S256(), b[:])
+		priv, _ := btcec.PrivKeyFromBytes(b[:])
 		*e = priv
 
 	case **btcec.PublicKey:
@@ -326,7 +326,7 @@ 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
 		}