specify somewhere in the documentation that "apt upgrade" is not supported

Originally created by @goupille on #16845 (Redmine)

from time to time we receive reports from users complaining that

apt update && apt upgrade

doesn’t work. sometime they are just mentioning it like it is the usual way to do when one is starting tails.

I think that it would be a good thing to mention somewhere that it is not supported nor advised to do that, maybe https://tails.boum.org/doc/first_steps/upgrade/, or in the FAQ

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