[FIX] calendar: fix wrong date for recurring event emails

Reproduce this bug

    - Install Calendar
    - Create a recurring event starting at the date of yesterday
      (later than your actual time)
    - Set the recurrency to every day, 1 time (today)
    - Click on the today's event
    - Add attendees > Invitations > Send mail
    - Run the mail queue manually (Technical > Scheduled actions)

    The mail you sent are for the date of yesterday
    The behavior is the same for the reminders

Cause

    The mail are sent with the `_send_mail_to_attendees` method
    who pass the attendee_id to the templates.

    The `attendee_id.event_id` is always the first one.

This commit pass the correct event to the template via the context.
I replaced only the dates values in the template.

closes odoo/odoo#40166

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>