Draft: nextcloud: Migrate to upstream updating solution

Nextcloud has it's own updater. With this release, we are switching from supplying packages for Nextcloud and maintaining those to a setup where we provide initial installation and then we let Nextcloud keep itself up to date all by itself (with the help from cron).

Before setup, only placeholder webpage that redirects to reForis is used. Migration from previous version should be covered.

One issues that this setup solves is downgrades that are not supported, but might happen when switching between branches.

CC @mvasilek but assigning to @ljelinek as he probably has most experience with Nextcloud and PHP.