Document upgrade process from 4.x to 7.x including pitfalls and best practices

[Svanvith]

We need a comprehensive and practical documentation of the upgrade process from OpenCloud 4.x to (likely) 6.x.

There is already an initial guide available here:
https://docs.opencloud.eu/docs/next/admin/maintainance/upgrade/

However, this should be expanded into a more complete, production-ready upgrade guide.

Goals:

Create a clear, step-by-step upgrade documentation that helps administrators perform the upgrade safely and efficiently.

Tasks:

• Add a structured, step-by-step guide including:

  • Preconditions (backups, version requirements, compatibility checks, etc.)

  • Recommended upgrade path (including intermediate versions if required)

  • Commands and configuration changes

• Identify and document common pitfalls and known issues, such as:

  • Breaking changes between versions
  • Deprecated or removed features
  • Required manual interventions
  • Configuration changes (e.g. env vars, services, dependencies)

• Include troubleshooting guidance for typical upgrade failures

• Add validation steps after upgrade (how to verify the system is working correctly)

[JammingBen]

From a web client perspective, admins need to update web extensions to their latest version, older versions won't work. New versions can be found in the app store in the OpenCloud UI or on https://github.com/opencloud-eu/web-extensions/releases.

[Heiko-Pohl]

I create a tutorial in PR: #863