[FIX] stock: correct validated inventory date

Currently the help of stock_inventory.date includes:
"If the inventory adjustment is validated, date at which the inventory
adjustment has been validated."

But for some reason this isn't the case. This commit makes it so we now
update the `date` when the inventory is validated. Since no one
previously complained about this, we only apply it to newly validated
inventories and leave existing ones as is. Note this change makes the
`accounting_date` help accurate (i.e generated account moves already
use the date when the inventory is validated when no accounting date
set, so before this fix the dates won't match).

Discovered during task: 2336455

closes odoo/odoo#59143

X-original-commit: 1da012147fa49045bb1dff25eb304364d47e0271
Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>

addons/stock/models/stock_inventory.py

@@ -123,7 +123,7 @@ class Inventory(models.Model):
                 negative.product_qty
             ))
         self.action_check()
-        self.write({'state': 'done'})
+        self.write({'state': 'done', 'date': fields.Datetime.now()})
         self.post_inventory()
         return True