Fixes #756 - Add upgrade instructions for Zammad when upgrading Debian 12 to 13

[ralf401]

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[mgruner]

Looks awesome! We'll need something similar for all distros when we switch to the new repo for all, in a later version. 👏

[MrGeneration]

Sorry for butting in:
https://github.com/zammad/zammad-documentation/pull/757/changes#diff-0636374e57518364fd7e243009c18af23161f22f6daa2588867dc03af5d26946R100

Executing apt upgrade -y with the new sources, will cause Zammad to be updated (and started) during the distribution upgrade. Wouldn't a two phased upgrade better here? E.g. doing that after rebooting? Because you can differentiate issues with the upgrade easier that way and your backup stays consistent?

Personally I prefer to hold the package during such an upgrade.
Additionally, usually you ensure that your packages are at the latest version available before changing your repos to a newer version. Especially relevant for admins updating with much slower pace than others (of course with holded Zammad package).

Apart from that, nice work.

[ralf401]

@MrGeneration Thanks for your hints. I added a general hint about that and also referred to the default "Update Zammad" page, where some additional information is present (e.g. not updating from very old to very new).

@mgruner Thanks for your review! In addition to the above mentioned, I just added a sentence to reboot the device after the update. Hope this is fine and I'll merge it then. In case you have any objections, please let me know.