Fix import for file dialog

Without this, the page fails to load, showing nothing, because the FileDialog element doesn't exist.
I don't know why this is necessary for Qt6, but we had to do this for all other FileDialog uses too.

Done as a 5-minute fix.

plugins/FirmwareUpdater/FirmwareUpdaterMachineAction.qml

@@ -5,7 +5,7 @@ import QtQuick 2.2
 import QtQuick.Controls 2.1
 import QtQuick.Layouts 1.1
 import QtQuick.Window 2.1
-import QtQuick.Dialogs 6.0 // For filedialog
+import QtQuick.Dialogs // For filedialog
 
 import UM 1.5 as UM
 import Cura 1.0 as Cura