Move Debian and Ubuntu requirements at the beginning of /install/debian/usb

As a follow-up on #11015 (closed) and #11012 (closed), I think people should be warned about the version of Debian and Ubuntu needed to follow these scenarios only once and at the beginning of the scenario.

I’m surprised I didn’t think about this earlier :)

Feature Branch: web/11017-debian-requirements-first

Related issues

Related to #11015 (closed)
Related to #11012 (closed)

Original created by @sajolida on 11017 (Redmine)

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