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

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

@@ -1146,6 +1146,12 @@ Superuser created successfully.
           will no longer work and must be updated.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          Nextcloud 20 (<literal>pkgs.nextcloud20</literal>) has been
+          dropped because it was EOLed by upstream in 2021-10.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
   <section xml:id="sec-release-21.11-notable-changes">

nixos/doc/manual/release-notes/rl-2111.section.md

@@ -354,6 +354,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 - The `services.mosquitto` module has been rewritten to support multiple listeners and per-listener configuration.
   Module configurations from previous releases will no longer work and must be updated.
 
+- Nextcloud 20 (`pkgs.nextcloud20`) has been dropped because it was EOLed by upstream in 2021-10.
+
 ## Other Notable Changes {#sec-release-21.11-notable-changes}