luci-proto-openconnect: fix certificate file paths

This addresses the issue of openconnect.sh from openconnect package expecting
a vpn- suffix for the files, while the frontend didn't.

Signed-off-by: Friendly fellow <DasTestament@users.noreply.github.com>
[reword commit message]
Ref: https://github.com/openwrt/packages/issues/11584
Signed-off-by: Jo-Philipp Wich <jo@mein.io>