Investigate the new API/Framework and possibility to retire SecureTransport backend

Apple has deprecated their TLS implementation (aka SecureTransport/and maybe CoreTLS?) since 10.14+. Instead, they recommend using their 'Network' framework (https://developer.apple.com/documentation/network?language=objc, note they only recently updated the docs) which is, while quite different from what SecureTransport is/was, can be quite handy in implementing a new QSslSocket back-end, albeit using a bit different level of abstraction. In the past, because of Apple ditching OpenSSL, we were using NSUrlConnection, which is quite high level abstraction and is a QNAM of its own. We got rid of it after a SecureTransport back-end was contributed by Jeremy Lane (kudos and many thanks btw). But now Apple does a bad thing to us again - they deprecated SecureTransport. This "Network" framework though seems to be closer (if comparing it to NSUrlConnection) to what we need to implement QSslSocket (and apparently is using a good TLS implementation under the hood, like ... OpenSSL or its fork). So, back to the task: it makes a lot of sense to investigate if we can have such a new back-end and get rid of SecureTransport, which was always a handicapped rubbish in the end.