Don't use setFilterRegularExpression() in Qt 5

Otherwise showing child nodes when their parent matches does not work with
Qt 5. Interestingly it *does* work with Qt 6. That's especially strange
because the code of the filter model responsible for showing child nodes
when their parents match should work completely independent of the
particular filter being used.

gui/mainwindow.cpp

@@ -1002,7 +1002,7 @@ void MainWindow::removeEntry()
 void MainWindow::applyFilter(const QString &filterText)
 {
     m_entryFilterModel->
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 12, 0))
+#if (QT_VERSION >= QT_VERSION_CHECK(6, 0, 0))
         setFilterRegularExpression
 #else
         setFilterRegExp