qt, refactor: Fix 'QDateTime is deprecated' warnings

src/qt/guiutil.cpp

@@ -920,4 +920,13 @@ void PopupMenu(QMenu* menu, const QPoint& point, QAction* at_action)
     menu->popup(point, at_action);
 }
 
+QDateTime StartOfDay(const QDate& date)
+{
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
+    return date.startOfDay();
+#else
+    return QDateTime(date);
+#endif
+}
+
 } // namespace GUIUtil