security/openvpn[-mbedtls] update to OpenVPN 2.4.7

Upstream release announcement:
"This is primarily a maintenance release with bugfixes and improvements.
One of the big things is enhanced TLS 1.3 support

Please note that LibreSSL is not a supported crypto backend. We accept
patches and we do test on OpenBSD 6.0 which comes with LibreSSL, but if
newer versions of LibreSSL break API compatibility we do not take
responsibility to fix that."

Move USES up to please portlint.

Change summary:
https://github.com/OpenVPN/openvpn/blob/release/2.4/Changes.rst#version-247

Detailed change list:
https://community.openvpn.net/openvpn/wiki/ChangesInOpenvpn24#OpenVPN2.4.7