[IMP] purchase: do not make the field required

The value is made by the computed field.
Setting one has no effect (no inverse method).

If you have no value on a purchase order, you end up in the confusing situation
where you can not save a new PO without setting a value to date_planned but,
after saving, the value is reset (no lines).

Do not block the creation of a new purchase order without lines

Fixes #23275

addons/purchase/views/purchase_views.xml

@@ -264,7 +264,7 @@
                                 <group>
                                     <label for="date_planned"/>
                                     <div>
-                                        <field name="date_planned" required="1" attrs="{'readonly': [('state', 'not in', ('draft', 'sent'))]}"/>
+                                        <field name="date_planned" attrs="{'readonly': [('state', 'not in', ('draft', 'sent'))]}"/>
                                         <button name="action_set_date_planned" type="object"
                                             states="draft,sent"
                                             string="Set date to all order lines"