[FIX] event[_sms]: send emails direclty after attendee confirmation

Steps to reproduce:

  - Install `Events` module
  - Create a new event and set it to `Autoconfirmation`
  - Create a new attendee and confirm it

Issue:

  Mail that confirm registration is not sent to attendee.

Cause:

  When computing the `schedule_date`, it take the value of the
  `registration_id.create_date` WITH the microseconds.

  When creating a new attendee, at some point we run the mail schdelure
  that compare if the `schedule_date <= now`.
  `schedule_date` and `now` have same value except for the microseconds
  (set on `schedule_date` but not on `now`).
  Therefore `schedule_date > now` and the mail is not sent.

Solution:

  Remove microseconds from the schedule_date.

opw-3079389

closes odoo/odoo#122137

X-original-commit: f2adca45dcec5a11daa92d07bae00615210ea0de
Signed-off-by: Nasreddin Boulif (bon) <bon@odoo.com>