Fix #7997: describe how to upgrade Salt Minion in a proper way

[vutny]

What does this PR do?

Updates FAQ with Salt Minion upgrade techniques using modern Salt facilities and hacky workarounds for Debian.

What issues does this PR fix or reference?

Should resolve issue #7997

[cachedout]

Great! Thanks for taking this one on, @vutny

[F30]

I experienced some issues deploying the new solution: The Master loses connection to the Minion upon restart, which makes it print a "Minion did not return. [No response]" error and probably also aborts the highstate run for that Minion. I fixed this by adding order: last, which is what I also had been using with the previous (nohup-based) solution.

Two additional notes:

• Contrary to what the new heading suggests, this solution is not only helpful for restarts after upgrades, but for all restarts, e.g. also after changes to the Minion config.
• A similar procedure also has to be deployed for restarting the Salt Master.