refactor(journald): migrate retention setting

modules/services/maintenance.nix

@@ -9,5 +9,7 @@
     options = "--delete-older-than 14d";
   };
 
-  services.journald.extraConfig = "SystemMaxUse=500M";
+  services.journald.settings.Journal = {
+    SystemMaxUse = "500M";
+  };
 }