security/openvpn-devel: fix mbedTLS3 bootstrapping

and switch to depend on the net/mbedtls3 port,
as we no longer carry mbedtls2 in ports.

Also, mbedTLS 3 supports TLSv1.3, so drop our local MBEDTLS_DESC
and go with the official description instead.

Approved by: Gert Doering (maintainer, via IRC)
Related to:
PR: 289315