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/chanvalidate/validate.go

@@ -4,9 +4,9 @@ import (
 	"bytes"
 	"fmt"
 
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 

lnwallet/chanvalidate/validate_test.go

@@ -4,11 +4,12 @@ import (
 	"bytes"
 	"testing"
 
-	"github.com/btcsuite/btcd/btcec"
+	"github.com/btcsuite/btcd/btcec/v2"
+	"github.com/btcsuite/btcd/btcec/v2/ecdsa"
+	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
-	"github.com/btcsuite/btcutil"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
@@ -27,8 +28,8 @@ var (
 		0x69, 0x49, 0x18, 0x83, 0x31, 0x98, 0x47, 0x53,
 	}
 
-	alicePriv, alicePub = btcec.PrivKeyFromBytes(btcec.S256(), aliceKey[:])
-	bobPriv, bobPub     = btcec.PrivKeyFromBytes(btcec.S256(), bobKey[:])
+	alicePriv, alicePub = btcec.PrivKeyFromBytes(aliceKey[:])
+	bobPriv, bobPub     = btcec.PrivKeyFromBytes(bobKey[:])
 )
 
 // channelTestCtx holds shared context that will be used in all tests cases
@@ -106,9 +107,7 @@ func newChannelTestCtx(chanSize int64) (*channelTestCtx, error) {
 		return nil, err
 	}
 
-	aliceSig, err := btcec.ParseDERSignature(
-		aliceSigRaw, btcec.S256(),
-	)
+	aliceSig, err := ecdsa.ParseDERSignature(aliceSigRaw)
 	if err != nil {
 		return nil, err
 	}
@@ -121,9 +120,7 @@ func newChannelTestCtx(chanSize int64) (*channelTestCtx, error) {
 		return nil, err
 	}
 
-	bobSig, err := btcec.ParseDERSignature(
-		bobSigRaw, btcec.S256(),
-	)
+	bobSig, err := ecdsa.ParseDERSignature(bobSigRaw)
 	if err != nil {
 		return nil, err
 	}