security/openvpn*: update to 2.6.0, keep openvpn25

• copy openvpn to openvpn25, mark as deprecated and to expire March 31

• update openvpn to openvpn 2.6.0, highlights from Frank Lichtenheld's release announcement e-mail, slightly edited:
  • Data Channel Offload (DCO) kernel acceleration support for Windows, Linux, and FreeBSD [14].
  • OpenSSL 3 support
  • Improved handling of tunnel MTU, including support for pushable MTU.
  • Outdated cryptographic algorithms disabled by default, but there are options to override if necessary.
  • Reworked TLS handshake, making OpenVPN immune to replay-packet state exhaustion attacks.
  • Added --peer-fingerprint mode for a more simplistic certificate setup and verification.
  • Improved protocol negotiation, leading to faster connection setup.

ChangeLog: https://github.com/OpenVPN/openvpn/blob/v2.6.0/Changes.rst