`Secure Transport` is deprecated & doesn't support TLS 1.3, and neither does `LibreSSL`

I think `curl-rust` should stick to `OpenSSL` on macOS and Linux by default. Despite `Secure Transport` and `LibreSSL` being "native" on macOS, they are not the preferred APIs. TLS 1.3 only works out of the box in the new `Network.framework` and there is no `curl` backend for it. The system-provided `libcurl` on macOS now uses `LibreSSL` so it's not even `Secure Transport` anymore.