[whoalin1]

Summary

Closes #8224

Snap upgrades are channel-scoped. Document that admins must snap switch --channel=… before a major bump, then snap refresh, and finish with sudo nextcloud.occ upgrade (not a host php occ as www-data).

Test plan

• Docs build for package_upgrade.rst
• Channel example matches snap release-strategy wiki naming

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AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.

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