Add warning for deprecated admin module

clanModules/admin/default.nix

@@ -0,0 +1,12 @@
+{ config, ... }:
+{
+  config.assertions = [
+    {
+      assertion = config.clan.inventory.services.admin != { };
+      message = ''
+        The admin module has been migrated from `clan.services` to `clan.instances`
+        See https://docs.clan.lol/TODO for updated usage.
+      '';
+    }
+  ];
+}