[FIX] web: Allow user to groupby nonstored field

For the moment we cannot groupby an inherited field (which is therefore related) in a graph view, so it is counted as being non-stored.
Because in the `_normalize` function we do not process fields that are not stored, whereas like here

https://github.com/odoo/odoo/blob/d679cd0d8ba9a2420e81a42a698763e9be2327e1/addons/web/static/src/legacy/js

 /control_panel/groupby_menu.js#L45-L49.

Typically when a field has store=False but sortable=True it is inherited from a stored field, so it can be sortable
So we have to use `sortable` instead of `sort` to make it consistent.

opw-2733133

closes odoo/odoo#83390

Related: odoo/enterprise#24315
Signed-off-by: Achraf <abz@odoo.com>