Replace obsolete functions of QSslSocket

The QSslSocket::setDefaultCaCertificates() and 
QSslSocket::systemCaCertificates() member functions are obsolete since 
Qt 5.12.

Author: hebasto

src/qt/paymentserver.cpp

@@ -41,8 +41,8 @@
 #include <QNetworkReply>
 #include <QNetworkRequest>
 #include <QSslCertificate>
+#include <QSslConfiguration>
 #include <QSslError>
-#include <QSslSocket>
 #include <QStringList>
 #include <QTextDocument>
 #include <QUrlQuery>
@@ -448,9 +448,9 @@ void PaymentServer::LoadRootCAs(X509_STORE* _store)
 
         certList = QSslCertificate::fromPath(certFile);
         // Use those certificates when fetching payment requests, too:
-        QSslSocket::setDefaultCaCertificates(certList);
+        QSslConfiguration::defaultConfiguration().setCaCertificates(certList);
     } else
-        certList = QSslSocket::systemCaCertificates();
+        certList = QSslConfiguration::systemCaCertificates();
 
     int nRootCerts = 0;
     const QDateTime currentTime = QDateTime::currentDateTime();