[IMP] various: Change 'assignation' to 'assignment' across all modules

Purpose
=======
The main definition of 'assignation' is the following:
   "an appointment to meet someone in secret, typically one made by lovers"

The correct word we should be using is 'assignment':
   "the allocation of someone or something as belonging to a particular group or category"

So in this commit change every iteration of the word
'assignation' to 'assignment', across all modules.

closes odoo/odoo#45041

Taskid: 1966215
Closes: #45041
Related: odoo/enterprise#8375
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>