[FIX] stock: cast string as date

Writing for the web interface on the scheduled date of a stock move
will failed as the date is send to the python server as a string. Then,
a duration in datetime format is added. This failed as operation on string
and datetime is forbidden.
This commit cast the string date into a datetime object before applying
date logic.

closes odoo/odoo#41916

Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>

addons/stock/models/stock_move.py

@@ -462,7 +462,7 @@ class StockMove(models.Model):
         elif vals.get('state', '') == 'done' and vals.get('date'):
             propagated_date_field = 'date'
         if propagated_date_field:
-            new_date = vals.get(propagated_date_field)
+            new_date = fields.Datetime.to_datetime(vals.get(propagated_date_field))
             for move in self:
                 move_dest_ids = move.move_dest_ids.filtered(lambda m: m.state not in ('done', 'cancel'))
                 if not move_dest_ids: