udev: downgrade log level about failure in notification

These may fail on shutdown/reboot, as the notification socket is already
closed.

Closes #39943.

src/udev/udev-config.c

@@ -557,9 +557,10 @@ int manager_serialize_config(Manager *manager) {
         if (r < 0)
                 return log_warning_errno(r, "Failed to finalize serialization file: %m");
 
+        /* This may fail on shutdown/reboot. Let's not warn louder. */
         r = notify_push_fd(fileno(f), "config-serialization");
         if (r < 0)
-                return log_warning_errno(r, "Failed to push serialization fd to service manager: %m");
+                return log_debug_errno(r, "Failed to push serialization fd to service manager: %m");
 
         log_debug("Serialized configurations.");
         return 0;

src/udev/udev-manager.c

@@ -1503,7 +1503,7 @@ int manager_main(Manager *manager) {
         /* We will start processing events in the loop below. Before starting processing, let's remove the
          * event serialization fd from the fdstore, to avoid retrieving the serialized events again in future
          * invocations. Otherwise, the serialized events may be processed multiple times. */
-        (void) notify_remove_fd_warn("event-serialization");
+        (void) notify_remove_fd("event-serialization");
 
         r = sd_event_loop(manager->event);
         if (r < 0)