security/openfortivpn: Update 1.21.0 => 1.23.1, take maintainership

Changelogs:
https://github.com/adrienverge/openfortivpn/blob/v1.23.1/CHANGELOG.md

Improve port:

• repalace PORTVERSION with DISTVERSION
• fix warnings from portclippy
• install config.sample: prevent rewrites user's config during every update
• remove useless config.template

PR: 287912
Co-authored-by: Ben Hutton <ben@benhutton.com.au>
MFH: 2025Q3