modules/version: remove default stateVersion value

The state version is a sensible value that should explicitly set in the
users config.

Following home-manager: https://github.com/nix-community/home-manager/pull/3043.

CHANGELOG.md

@@ -26,6 +26,9 @@
   name
 * Add html and man pages with all available options, see <https://t184256.github.io/nix-on-droid/>
   and `nix build github:t184256/nix-on-droid#manPages`
+* The option `system.stateVersion` does not have a default value anymore.
+  Previously, its default was `"19.09"` the release the state version was
+  introduced.
 
 ## Release 22.05
 

modules/version.nix

@@ -12,7 +12,6 @@ with lib;
 
     system.stateVersion = mkOption {
       type = types.enum [ "19.09" "20.03" "20.09" "21.05" "21.11" "22.05" ];
-      default = "19.09";
       description = ''
         It is occasionally necessary for Nix-on-Droid to change
         configuration defaults in a way that is incompatible with

pkgs/default.nix

@@ -35,6 +35,8 @@ let
         pkgs = pkgs.lib.mkForce pkgs; # to override ./modules/nixpkgs/config.nix
       };
 
+      system.stateVersion = "22.05";
+
       # Fix invoking bash after initial build.
       user.shell = "${initialPackageInfo.bash}/bin/bash";