Update Qt example app to Qt6

[PureTryOut]

It seems the current example app still uses Qt5. I did a quick (something like) s/Qt5/Qt6 in the CMake build and the build then fails:

/tmp/src/NymphCast-0.2-alpha0/player/NymphCastPlayer/mainwindow.cpp:870:20: error: 'class QTextStream' has no member named 'setCodec'
  870 |         textStream.setCodec("UTF-8");
      |                    ^~~~~~~~
/tmp/src/NymphCast-0.2-alpha0/player/NymphCastPlayer/mainwindow.cpp: In member function 'void MainWindow::appsListRefresh()':
/tmp/src/NymphCast-0.2-alpha0/player/NymphCastPlayer/mainwindow.cpp:1222:87: error: 'SkipEmptyParts' is not a member of 'QString'
 1222 |         QStringList appItems = (QString::fromStdString(appList)).split("\n", QString::SkipEmptyParts);
      |                                                                                       ^~~~~~~~~~~~~~

Qt5 however is EOL and should not be supported anymore.