[FIX] core: update cache on `parent_path` when updated

Purpose
=======

The parent_path is invalidated from cache when computing it from scratch
(with method `_parent_store_compute`) but not when it is recomputed
(method `_parent_store_update`).

The issue is that a computed field depending on 'parent_path'
won't receive the updated value using the cache, and could lead
to inconsistencies if the developer is not aware of that.

Specification
=============

On `_parent_store_update`, set the new value of 'parent_path' in the
cache before marking the records as modified.

task-2766452

closes odoo/odoo#84705

Signed-off-by: Raphael Collet <rco@odoo.com>
Co-authored-by: Rémy Voet <ryv@odoo.com>