Update copyright year to 2023

Update copyright year to 2023

Manually excluded ovpn_dco_win.h because it is an
imported file. ovpn_dco_linux.h is already excluded
because it still says 2021.

Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20230110160531.81010-1-frank@lichtenheld.com>
URL: https://patchwork.openvpn.net/project/openvpn2/patch/20230110160531.81010-1-frank@lichtenheld.com/
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit ccf9d57249acb9bc9a450aec3e613bda631415f5)