[FW][FIX] mail: use queue to send channel guidelines

Commit 775e8a9 forced channel guidelines to be sent directly. It seems
safer to use the cron queue as we do not see any evidence of daily use of
channel guidelines requiring immediate sending.

Task ID 2121319
PR #39986
FW PR #39978

addons/mail/models/mail_channel.py

@@ -424,7 +424,6 @@ class Channel(models.Model):
                 'recipient_ids': [(4, partner.id)]
             }
             mail = self.env['mail.mail'].create(create_values)
-            mail.send()
         return True
 
     def _update_moderation_email(self, emails, status):