version: remove default value

The user should always explicitly set the state version they wish to
use. Indeed, the configuration generated by the Home Manager install
script has set this option for a long time. This removal should
therefore not affect many users.

docs/release-notes/rl-2211.adoc

@@ -8,7 +8,15 @@ This is the current unstable branch and the information in this section is there
 
 This release has the following notable changes:
 
-* No changes.
+* The <<opt-home.stateVersion>> option no longer has a default value.
+It used to default to ``18.09'', which was the Home Manager version
+that introduced the option. If your configuration does not explicitly
+set this option then you need to add
++
+[source,nix]
+home.stateVersion = "18.09";
++
+to your configuration.
 
 [[sec-release-22.11-state-version-changes]]
 === State Version Changes

modules/misc/version.nix

@@ -17,7 +17,6 @@ with lib;
         "22.05"
         "22.11"
       ];
-      default = "18.09";
       description = ''
         It is occasionally necessary for Home Manager to change
         configuration defaults in a way that is incompatible with

tests/default.nix

@@ -14,8 +14,11 @@ let
       # Fix impurities. Without these some of the user's environment
       # will leak into the tests through `builtins.getEnv`.
       xdg.enable = true;
-      home.username = "hm-user";
-      home.homeDirectory = "/home/hm-user";
+      home = {
+        username = "hm-user";
+        homeDirectory = "/home/hm-user";
+        stateVersion = lib.mkDefault "18.09";
+      };
 
       # Avoid including documentation since this will cause
       # unnecessary rebuilds of the tests.