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

lntest/mock/secretkeyring.go

@@ -1,7 +1,8 @@
 package mock
 
 import (
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/lightningnetwork/lnd/keychain"
 )
@@ -45,7 +46,7 @@ func (s *SecretKeyRing) ECDH(_ keychain.KeyDescriptor,
 
 // SignMessage signs the passed message and ignores the KeyDescriptor.
 func (s *SecretKeyRing) SignMessage(_ keychain.KeyLocator,
-	msg []byte, doubleHash bool) (*btcec.Signature, error) {
+	msg []byte, doubleHash bool) (*ecdsa.Signature, error) {
 
 	var digest []byte
 	if doubleHash {
@@ -53,7 +54,7 @@ func (s *SecretKeyRing) SignMessage(_ keychain.KeyLocator,
 	} else {
 		digest = chainhash.HashB(msg)
 	}
-	return s.RootKey.Sign(digest)
+	return ecdsa.Sign(s.RootKey, digest), nil
 }
 
 // SignMessageCompact signs the passed message.
@@ -66,5 +67,5 @@ func (s *SecretKeyRing) SignMessageCompact(_ keychain.KeyLocator,
 	} else {
 		digest = chainhash.HashB(msg)
 	}
-	return btcec.SignCompact(btcec.S256(), s.RootKey, digest, true)
+	return ecdsa.SignCompact(s.RootKey, digest, true)
 }