[FIX] stock: Scheduled Date on transfers

When creating a transfer from:

Dashboard > Planned Transfer

The scheduled date is not configurable as explained in commit:a57a290bd89bee0c50c9c1b03e7185c0497e89ed

opw:1838620

addons/stock/views/stock_picking_views.xml

@@ -247,7 +247,7 @@
                             <field name="backorder_id" readonly="1" attrs="{'invisible': [('backorder_id','=',False)]}"/>
                         </group>
                         <group>
-                            <field name="scheduled_date" required="1"/>
+                            <field name="scheduled_date" attrs="{'readonly': [('id', '=', False)], 'required': [('id', '!=', False)]}"/>
                             <field name="origin" placeholder="e.g. PO0032"/>
                             <field name="owner_id" groups="stock.group_tracking_owner"/>
                             <div groups="stock.group_tracking_owner" colspan="2" col="2">