nextcloud: Review installation upgrade warning wording

The new wording does not assume the user is upgrading.

This is because a user could be setting up a new installation on 20.03
on a server that has a 19.09 or before stateVersion!!

The new wording ensures that confusion is reduced by stating that they
do not have to care about the assumed 16→17 transition.

Then, the wording explains that they should, and how to upgrade to
version 18.

It also reviews the confusing wording about "multiple" upgrades.

* * *

The only thing we cannot really do is stop a fresh install of 17 if
there was no previous install, as it cannot be detected. That makes a
useless upgrade forced for new users with old state versions.

It is also important to state that they must set their package to
Nextcloud 18, as future upgrades to Nextcloud will not allow an uprade
from 17!

I assume future warning messages will exist specifically stating what to
do to go from 18 to 19, then 19 to 20, etc...