nextcloud20: drop

The version 20 of Nextcloud will be EOLed by the end of this month[1].

Since the recommended default (that didn't raise an eval-warning) on
21.05 was Nextcloud 21, this shouldn't affect too many people.

In order to ensure that nobody does a (not working) upgrade across
several major-versions of Nextcloud, I replaced the derivation of
`nextcloud20` with a `throw` that provides instructions how to proceed.

The only case that I consider "risky" is a setup upgraded from 21.05 (or
older) with a `system.stateVersion` <21.11 and with
`services.nextcloud.package` not explicitly declared in its config. To
avoid that, I also left the `else-if` for `stateVersion < 21.03` which
now sets `services.nextcloud.package` to `pkgs.nextcloud20` and thus
leads to an eval-error. This condition can be removed
as soon as 21.05 is EOL because then it's safe to assume that only
21.11. is used as stable release where no Nextcloud <=20 exists that can
lead to such an issue.

It can't be removed earlier because then every `system.stateVersion <
21.11` would lead to `nextcloud21` which is a problem if `nextcloud19`
is still used.

[1] https://docs.nextcloud.com/server/20/admin_manual/release_schedule.html

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