GitHub Actions: remove Ubuntu 16.04 environment

GitHub Actions: remove Ubuntu 16.04 environment

Starting from 20th of September, GitHub has removed Ubuntu 16.04
environment from Actions. Safely remove it from our build script
where we already have Ubuntu 18.04 and 20.04.

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20210926061046.212-1-lstipakov@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22904.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>