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/nodes.go

@@ -6,7 +6,7 @@ import (
 	"net"
 	"time"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/kvdb"
 )
@@ -298,7 +298,7 @@ func deserializeLinkNode(r io.Reader) (*LinkNode, error) {
 	if _, err := io.ReadFull(r, pub[:]); err != nil {
 		return nil, err
 	}
-	node.IdentityPub, err = btcec.ParsePubKey(pub[:], btcec.S256())
+	node.IdentityPub, err = btcec.ParsePubKey(pub[:])
 	if err != nil {
 		return nil, err
 	}