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/networking/kresd.nix

@@ -140,7 +140,7 @@ in {
 
     # Try cleaning up the previously default location of cache file.
     # Note that /var/cache/* should always be safe to remove.
-    # TODO: remove later, probably between 20.09 and 21.03
+    # TODO: remove later, probably between 20.09 and 21.05
     systemd.tmpfiles.rules = [ "R /var/cache/kresd" ];
   };
 }