2e1455c4a100170c322165af3ecbbcd3004b2f9d Replace obsolete functions of QSslSocket (Hennadii Stepanov)
Pull request description:
The [`QSslSocket::setDefaultCaCertificates()`](https://doc.qt.io/qt-5/qsslsocket-obsolete.html#setDefaultCaCertificates) and [`QSslSocket::systemCaCertificates()`](https://doc.qt.io/qt-5/qsslsocket-obsolete.html#systemCaCertificates) member functions are [obsolete](https://doc.qt.io/qt-5.12/qsslsocket-obsolete.html) since Qt 5.12.
This PR replaces them, does not change behavior and keeps compatibility with [Qt 5.5.1](https://github.com/bitcoin/bitcoin/pull/15393).
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This is a backport of Core PR16708