multi: update to fn v2

sweep/fee_function_test.go

@@ -3,7 +3,7 @@ package sweep
 import (
 	"testing"
 
-	"github.com/lightningnetwork/lnd/fn"
+	"github.com/lightningnetwork/lnd/fn/v2"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/stretchr/testify/require"
 )