security/openvpn-devel: upgrade port to git commit 7b1b283478 (2.7_alpha3, 2025-07-31)

This commit brings the port to "openvpn 2.7_alpha3".

For FreeBSD, the most significant change is that "floating clients with
DCO" are supported, if the kernel has support for it (-current).

Platform-independent the "big new feature" is client side support
for PUSH_UPDATE (send new configuration data while a client-server
connection is established).