Forces curl to use TLS version 1.3 when connecting to a remote TLS server.
Note that TLS 1.3 is only supported by a subset of TLS backends. At the time
-of writing this, those are BoringSSL and NSS only.
+of this writing, they are BoringSSL, NSS, and Secure Transport (on iOS 11 or
+later, and macOS 10.13 or later).