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
{
    
    
    
    std::string platformName;
    platformName = gArgs.GetArg("-uiplatform", BitcoinGUI::DEFAULT_UIPLATFORM);
    platformStyle = PlatformStyle::instantiate(QString::fromStdString(platformName));
    if (!platformStyle) 
        platformStyle = PlatformStyle::instantiate("other");
    assert(platformStyle);
}
BitcoinApplication::~BitcoinApplication()
{
    if(coreThread)
    {
        qDebug() << __func__ << ": Stopping thread";
        coreThread->quit();
        coreThread->wait();
        qDebug() << __func__ << ": Stopped thread";
    }
    delete window;
    window = nullptr;
    delete platformStyle;
    platformStyle = nullptr;
}
#ifdef ENABLE_WALLET
void BitcoinApplication::createPaymentServer()