As branching `0.21.0` off is [approaching](https://github.com/bitcoin/bitcoin/issues/18947) it seems a good time to discuss a possible bumping of the minimum required Qt5 version.

The current minimum required Qt is 5.5.1. Set in #15393. Recent discussion in #13478.

Distro support:
- stretch -- [5.7.1](https://packages.debian.org/stretch/libqt5core5a)
- buster -- [5.11.3](https://packages.debian.org/buster/libqt5core5a)
- bionic -- [5.9.5](https://packages.ubuntu.com/bionic/libqt5core5a)
- focal -- [5.12.8](https://packages.ubuntu.com/focal/libqt5core5a)
- centos 7 -- [5.9.7](http://mirror.centos.org/centos/7/os/x86_64/Packages/qt5-qtbase-5.9.7-2.el7.x86_64.rpm)
- centos 8 -- [5.12.5](http://mirror.centos.org/centos/8/AppStream/x86_64/os/Packages/qt5-qtbase-5.12.5-4.el8.x86_64.rpm)

Bumping to 5.9 (LTS) is possible, but it won't be of much benefit to us. Also, its support [ended](https://www.qt.io/blog/2017/06/07/renewed-qt-support-services) on 2020-05-31.

5.10 could bring much more benefits to us, e.g.:
- functor-parameter overload of [`QMetaObject::invokeMethod`](https://doc.qt.io/qt-5/qmetaobject.html#invokeMethod-4)
- fixed https://bugreports.qt.io/browse/QTBUG-10907
{
#ifdef WIN32
    util::WinCmdLineArgs winArgs;
    std::tie(argc, argv) = winArgs.get();
#endif
    SetupEnvironment();
    util::ThreadSetInternalName("main");
    NodeContext node_context;
    std::unique_ptr<interfaces::Node> node = interfaces::MakeNode(&node_context);
    
    boost::signals2::scoped_connection handler_message_box = ::uiInterface.ThreadSafeMessageBox_connect(noui_ThreadSafeMessageBox);
    boost::signals2::scoped_connection handler_question = ::uiInterface.ThreadSafeQuestion_connect(noui_ThreadSafeQuestion);
    boost::signals2::scoped_connection handler_init_message = ::uiInterface.InitMessage_connect(noui_InitMessage);
    
    
    Q_INIT_RESOURCE(bitcoin);
    Q_INIT_RESOURCE(bitcoin_locale);
    
    QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
#if QT_VERSION >= 0x050600
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif
#if (QT_VERSION <= QT_VERSION_CHECK(5, 9, 8)) && defined(Q_OS_MACOS)
    const auto os_name = QSysInfo::prettyProductName();
    if (os_name.startsWith("macOS 11") || os_name.startsWith("macOS 10.16")) {
        QApplication::setStyle("fusion");
    }
#endif