qt6: Do not use deprecated high DPI attributes in Qt 6

In Qt 6, high DPI pixmaps and scaling are always enabled.

https://doc.qt.io/qt-6/highdpi.html

src/qt/bitcoin.cpp

@@ -530,9 +530,11 @@ int GuiMain(int argc, char* argv[])
     Q_INIT_RESOURCE(bitcoin);
     Q_INIT_RESOURCE(bitcoin_locale);
 
+#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0))
     // Generate high-dpi pixmaps
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+#endif
 
 #if defined(QT_QPA_PLATFORM_ANDROID)
     QApplication::setAttribute(Qt::AA_DontUseNativeMenuBar);