security/openvpn-devel: upgrade port to git commit 566c0791ca (2022-12-15)

this commit is also 2.6_beta2.

contains a number of bugfixes and minor improvements

user visible changes since last port upgrade

• freebsd + DCO now has working byte counters in DCO mode
• server no longer accepts new inbound client connections while in "explicit-exit-notify" shutdown phase
• logging improvements for DCO key handling