treewide: update 21.03 to 21.05

The NixOS 21.03 release has been delayed to 21.05. See NixOS/rfcs#80.

There are two instances of 21.03 which have been left as is, since they
are in stateVersion comparisons. This will ensure that existing user
configurations which refer to 21.03 will continue to work.

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

@@ -368,7 +368,7 @@ in {
         '')
         ++ (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"));
+        ++ (optional (versionOlder cfg.package.version "20") (upgradeWarning 19 "21.05"));
 
       services.nextcloud.package = with pkgs;
         mkDefault (
@@ -380,6 +380,10 @@ in {
             ''
           else if versionOlder stateVersion "20.03" then nextcloud17
           else if versionOlder stateVersion "20.09" then nextcloud18
+          # 21.03 will not be an official release - it was instead 21.05.
+          # This versionOlder statement remains set to 21.03 for backwards compatibility.
+          # See https://github.com/NixOS/nixpkgs/pull/108899 and
+          # https://github.com/NixOS/rfcs/blob/master/rfcs/0080-nixos-release-schedule.md.
           else if versionOlder stateVersion "21.03" then nextcloud19
           else nextcloud20
         );