openvpn: 2.4.5 -> 2.4.6

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/openvpn/versions.

These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.4.6 with grep in /nix/store/5hj70y409c0b01zmx4rddiicgq7jajb2-openvpn-2.4.6
- directory tree listing: https://gist.github.com/ce0bc5e31d7d26ead341febdc7bdc6ee