security/openvpn-devel: upgrade port to git commit 91b057a2b5 (2024-02-11)

contains a number of bugfixes and minor improvements, especially adding
more unit tests.

There is one FreeBSD relevant bugfix for servers with DCO, where reporting
of kernel peer statistics to userland failed due to static buffers sized
too small (ENOSPC). Buffer handling here has been made dynamic.

PR: 276973