Disable TLS 1.3 support with mbed TLS

Disable TLS 1.3 support with mbed TLS

As of version 3.5.0 the TLS-Exporter function is not yet implemented in
mbed TLS, and the exporter_master_secret is not exposed to the
application either. Falling back to an older PRF when claiming to use
TLS1.3 seems like false advertising.

Change-Id: If4e1c4af9831eb1090ccb3a3c4d3e76b413f0708
Signed-off-by: Max Fillinger <maximilian.fillinger@foxcrypto.com>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20231115151740.23948-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg27453.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit efad93d049c318a3bd9ea5956c6ac8237b8d6d70)