nixos/nextcloud: update upgrade-path for `nextcloud20`

Please note that this is only for 21.03 since `nextcloud19` is intended
to be the default for the already feature-frozen 20.09 (the bump itself
is supposed to get backported however).

nixos/doc/manual/release-notes/rl-2103.xml

@@ -75,7 +75,16 @@
 
   <itemizedlist>
    <listitem>
-    <para />
+    <para>
+     The default-version of <literal>nextcloud</literal> is <package>nextcloud20</package>.
+     Please note that it's <emphasis>not</emphasis> possible to upgrade <literal>nextcloud</literal>
+     across multiple major versions! This means that it's e.g. not possible to upgrade
+     from <package>nextcloud18</package> to <package>nextcloud20</package> in a single deploy.
+    </para>
+    <para>
+     The package can be manually upgraded by setting <xref linkend="opt-services.nextcloud.package" />
+     to <package>nextcloud20</package>.
+    </para>
    </listitem>
   </itemizedlist>
  </section>

nixos/modules/services/web-apps/nextcloud.nix

@@ -330,37 +330,28 @@ in {
         }
       ];
 
-      warnings = []
-        ++ (optional (cfg.poolConfig != null) ''
-          Using config.services.nextcloud.poolConfig is deprecated and will become unsupported in a future release.
-          Please migrate your configuration to config.services.nextcloud.poolSettings.
-        '')
-        ++ (optional (versionOlder cfg.package.version "18") ''
-          A legacy Nextcloud install (from before NixOS 20.03) may be installed.
-
-          You're currently deploying an older version of Nextcloud. This may be needed
-          since Nextcloud doesn't allow major version upgrades that skip multiple
-          versions (i.e. an upgrade from 16 is possible to 17, but not 16 to 18).
-
-          It is assumed that Nextcloud will be upgraded from version 16 to 17.
+      warnings = let
+        latest = 20;
+        upgradeWarning = major: nixos:
+          ''
+            A legacy Nextcloud install (from before NixOS ${nixos}) may be installed.
 
-           * If this is a fresh install, there will be no upgrade to do now.
+            After nextcloud${toString major} is installed successfully, you can safely upgrade
+            to ${toString (major + 1)}. The latest version available is nextcloud${toString latest}.
 
-           * If this server already had Nextcloud installed, first deploy this to your
-             server, and wait until the upgrade to 17 is finished.
+            Please note that Nextcloud doesn't support upgrades across multiple major versions
+            (i.e. an upgrade from 16 is possible to 17, but not 16 to 18).
 
-          Then, set `services.nextcloud.package` to `pkgs.nextcloud18` to upgrade to
-          Nextcloud version 18. Please note that Nextcloud 19 is already out and it's
-          recommended to upgrade to nextcloud19 after that.
+            The package can be upgraded by explicitly declaring the service-option
+            `services.nextcloud.package`.
+          '';
+      in (optional (cfg.poolConfig != null) ''
+          Using config.services.nextcloud.poolConfig is deprecated and will become unsupported in a future release.
+          Please migrate your configuration to config.services.nextcloud.poolSettings.
         '')
-        ++ (optional (versionOlder cfg.package.version "19") ''
-          A legacy Nextcloud install (from before NixOS 20.09) may be installed.
-
-          If/After nextcloud18 is installed successfully, you can safely upgrade to
-          nextcloud19. If not, please upgrade to nextcloud18 first since Nextcloud doesn't
-          support upgrades that skip multiple versions (i.e. an upgrade from 17 to 19 isn't
-          possible, but an upgrade from 18 to 19).
-        '');
+        ++ (optional (versionOlder cfg.package.version "18") (upgradeWarning 17 "20.03"))
+        ++ (optional (versionOlder cfg.package.version "19") (upgradeWarning 18 "20.09"))
+        ++ (optional (versionOlder cfg.package.version "20") (upgradeWarning 19 "21.03"));
 
       services.nextcloud.package = with pkgs;
         mkDefault (
@@ -372,7 +363,8 @@ in {
             ''
           else if versionOlder stateVersion "20.03" then nextcloud17
           else if versionOlder stateVersion "20.09" then nextcloud18
-          else nextcloud19
+          else if versionOlder stateVersion "21.03" then nextcloud19
+          else nextcloud20
         );
     }