Revert "package/openvpn: bump version to 2.3.10"

openvpn 2.3.10 doesn't work with polarssl 1.2.x, hence this bump breaks
builds for the polarssl backend.

This reverts commit 06f3e790.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>