Warn admin about outdated Collabora versions

We notice quite often that users update their Nextcloud but have no information that there is a new Collabora version available (both patch updates but also major versions).

On the Nextcloud side we can send notifications to admins and also warn them prominently in the admin overview.

While Collabora has a hint about new versions it is only shown to admins when they use collabora which might not happen often and therefore might be unseen.

My proposal would be to extend this in the richdocuments app:

• Regularly check if there is a new Collabora version available
• Notify admins about it
• Show a warning in the admin overview of Nextcloud

We can read the currently installed version from capabilities of Collabora but there is no way I'm aware to know about the latest published version in machine readable form (both for CODE and COOL releases).

@pedropintosilva Not sure who to ping, but it would be nice to just have some endpoint that we can query that will tell us the latest version tag for CODE / COOL.

Start by inspecting the richdocuments app's existing handling of Collabora capabilities and the Nextcloud admin notification and overview integration points. Confirm how the currently installed version is read, then investigate the proposed machine-readable CODE/COOL release endpoint. Done means admins are regularly informed about newer versions and see a warning in the admin overview, subject to an agreed version source.