core: downgrade log message about inability to propagate cgroup release message

If dbus is already down during shutdown, we can't propagate the cgroup
release message anymore, but that's expected and nothing to warn about.
Hence let's downgrade the message from LOG_WARN to LOG_DEBUG.

Fixes: #6777

src/core/dbus.c

@@ -92,7 +92,7 @@ int bus_forward_agent_released(Manager *m, const char *path) {
                                "Released",
                                "s", path);
         if (r < 0)
-                return log_warning_errno(r, "Failed to propagate agent release message: %m");
+                return log_debug_errno(r, "Failed to propagate agent release message: %m");
 
         return 1;
 }

src/core/manager.c

@@ -1780,7 +1780,7 @@ static int manager_dispatch_cgroups_agent_fd(sd_event_source *source, int fd, ui
         buf[n] = 0;
 
         manager_notify_cgroup_empty(m, buf);
-        bus_forward_agent_released(m, buf);
+        (void) bus_forward_agent_released(m, buf);
 
         return 0;
 }