[backport] qt: Replace obsolete functions of QSslSocket #16708

Summary:
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 since Qt 5.12.

This PR replaces them, does not change behavior and keeps compatibility with Qt 5.5.1.

This is a backport of Core PR16708

Test Plan:

ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5382