move system.stateVersion back to userdata

configuration.nix

@@ -78,6 +78,7 @@
     "# Completely remove remnants of NIXOS_LUSTRATE."
     "R! /old-root"
   ];
+  system.stateVersion = config.selfprivacy.stateVersion;
   system.autoUpgrade = {
     enable = config.selfprivacy.autoUpgrade.enable;
     allowReboot = config.selfprivacy.autoUpgrade.allowReboot;

selfprivacy-module.nix

@@ -34,6 +34,10 @@ with lib;
         type = types.nullOr types.bool;
       };
     };
+    stateVersion = mkOption {
+      description = "State version of the server";
+      type = types.str;
+    };
     ########################
     # Server admin options #
     ########################