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/btcwallet/psbt_test.go

@@ -7,12 +7,12 @@ import (
 	"fmt"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcutil"
+	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/btcsuite/btcd/chaincfg"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
-	"github.com/btcsuite/btcutil/psbt"
 	"github.com/btcsuite/btcwallet/waddrmgr"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
@@ -29,7 +29,7 @@ var (
 			"aca5",
 	)
 	testCommitSecret, testCommitPoint = btcec.PrivKeyFromBytes(
-		btcec.S256(), testCommitSecretBytes,
+		testCommitSecretBytes,
 	)
 
 	remoteRevocationBasePubKeyBytes, _ = hex.DecodeString(
@@ -37,7 +37,7 @@ var (
 			"7359fb",
 	)
 	remoteRevocationBasePubKey, _ = btcec.ParsePubKey(
-		remoteRevocationBasePubKeyBytes, btcec.S256(),
+		remoteRevocationBasePubKeyBytes,
 	)
 
 	testTweakSingle, _ = hex.DecodeString(