[REF] survey: add a "_mark_done" method to improve answers state management

As other modules than survey (namely hr_skills) now need a hook to add specific
behavior when a survey answer is marked as 'done', we removed the '_send_certification'
method and replaced it with a '_mark_done' method.

It will make it easier to override this method as it has a more generic meaning.

closes odoo/odoo#32880

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>