Qt6 port:

* QApplication::setFallbackSessionManagementEnabled has been removed
* QString::medRef() has been removed. Use QString::mid() again.
* QTextStream::setCodec has been removed
* Use operator QVariant of the QFont class to make code Qt5 and Qt6 compatible
* Signature of QTreeWidget::mimeData() has changed in Qt6. Remove TreeWidget::mimeData() because it doesn't change the implementation
* QLayout::setMargin() is deprecated in Qt5 and has been removed in Qt6. Use QLayout::setContentsMargins()
* QDateTime::toTime_t() is deprecated in Qt5 and has been removed in Qt6. Use QDateTime::toSecsSinceEpoch()
* QDesktopWidget is deprecated in Qt5 and has been removed in Qt6. Use QScreen

src/Gui/DocumentRecovery.cpp

@@ -355,7 +355,9 @@ void DocumentRecoveryPrivate::writeRecoveryInfo(const DocumentRecoveryPrivate::I
     QFile file(info.xmlFile);
     if (file.open(QFile::WriteOnly)) {
         QTextStream str(&file);
+#if QT_VERSION < QT_VERSION_CHECK(6,0,0)
         str.setCodec("UTF-8");
+#endif
         str << "<?xml version='1.0' encoding='utf-8'?>\n"
             << "<AutoRecovery SchemaVersion=\"1\">\n";
         switch (info.status) {