security/openvpn-devel: upgrade port to git commit 480ad2a84e (2023-01-13)

this commit is also 2.6_rc2 (+ a freebsd/linux build fix).

contains a number of bugfixes and minor improvements, see
https://github.com/OpenVPN/openvpn/blob/v2.6_rc2/Changes.rst

Most notable improvement: this contains a default-enabled ratelimiter
for initial TLS handshake packets (100 per 10s), significantly reducing
the usefulness of abusing an OpenVPN server as a DDoS reflection engine.

Use '--connect-freq-initial' to tune parameters if needed.

(cherry picked from commit 0ffbe3c4015a5c04bf8b6754f7cdf396262e931f)