nixos/stalwart: switch logging from stdout to journal

journald's centralized logging has many advantages:
- External centralized logging can simply source from journald
- journald's auto-vacuum options (e.g. SystemMaxUse) can make sure logs
  don't take up too much space
- Logs are in a single timeline, which is useful for analysis

nixos/doc/manual/release-notes/rl-2605.section.md

@@ -168,6 +168,7 @@ of pulling the upstream container image from Docker Hub. If you want the old beh
 
   - `systemd.services.stalwart` owned by `stalwart:stalwart`. The `user` and `group` are configurable via `services.stalwart.user` and `services.stalwart.group`, respectively. By default, if `stateVersion` is older than `26.05`, will fallback to legacy value of `stalwart-mail` for both `user` and `group`.
   - Default value for `services.stalwart.dataDir` has changed to `/var/lib/stalwart`. If `stateVersion` is older than `26.05`, will fallback to legacy value of `/var/lib/stalwart-mail`.
+  - Default tracer name and type have changed to `journal`. If `stateVersion` is older than `26.05`, will fallback to legacy value of `stdout`.
 
 - `services.eintopf` has been renamed to `services.lauti` to align with upstream re-brand as a community online calendar.
 

nixos/modules/services/mail/stalwart.nix

@@ -9,8 +9,8 @@ let
   configFormat = pkgs.formats.toml { };
   configFile = configFormat.generate "stalwart.toml" cfg.settings;
   useLegacyStorage = lib.versionOlder cfg.stateVersion "24.11";
-  useLegacyIdentifier = lib.versionOlder cfg.stateVersion "26.05";
-  stalwartIdentifier = if useLegacyIdentifier then "stalwart-mail" else "stalwart";
+  pre2605 = lib.versionOlder cfg.stateVersion "26.05";
+  stalwartIdentifier = if pre2605 then "stalwart-mail" else "stalwart";
 
   parsePorts =
     listeners:
@@ -123,12 +123,24 @@ in
 
     # Default config: all local
     services.stalwart.settings = {
-      tracer.stdout = {
-        type = lib.mkDefault "stdout";
-        level = lib.mkDefault "info";
-        ansi = lib.mkDefault false; # no colour markers to journald
-        enable = lib.mkDefault true;
-      };
+      tracer =
+        if pre2605 then
+          {
+            stdout = {
+              type = lib.mkDefault "stdout";
+              level = lib.mkDefault "info";
+              ansi = lib.mkDefault false; # no colour markers to journald
+              enable = lib.mkDefault true;
+            };
+          }
+        else
+          {
+            journal = {
+              type = lib.mkDefault "journal";
+              level = lib.mkDefault "info";
+              enable = lib.mkDefault true;
+            };
+          };
       store =
         if useLegacyStorage then
           {