fix #15127 ActiveJob::DeserializationError thrown

`send_devise_notification` pre-maturely enqueued the task when the user instance
has not yet been committed into the database, causing a record-not-found in the
other sidekiq process.

`devise-async` has already been taking care of asynchronous mail sending, we just
need to run it inside queue `mailers` instead of `mailer` to enable it.

The implementation of `devise-async` enqueues the task in `after_commit` hook
which is the right way to do it.

See merge request !3647