GHA: Update mbedtls to v4

GHA: Update mbedtls to v4

Also switch from Make to CMake for building it
because the former is not supported anymore.

Change-Id: I658b1b24da304938225a8f834d7484671a63360f
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Yuriy Darnobyt <yura.uddr@gmail.com>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1474
Message-Id: <20260124181814.30331-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg35421.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>