Set the invoice date to read-only once the invoice has been validated or posted

Closes #14480
(grafted from 131cabff89d9e3f130cc29dd800fedc0997d4ac9)

--HG--
branch : 6.0

modules/account_invoice/invoice.py

@@ -93,7 +93,10 @@ class Invoice(Workflow, ModelSQL, ModelView, TaxableMixin):
             ], "State", readonly=True)
     invoice_date = fields.Date('Invoice Date',
         states={
-            'readonly': Eval('state').in_(['posted', 'paid', 'cancelled']),
+            'readonly': Eval('state').in_(
+                If(Eval('type') == 'in',
+                    ['validated', 'posted', 'paid'],
+                    ['posted', 'paid'])),
             'required': Eval('state').in_(
                 If(Eval('type') == 'in',
                     ['validated', 'posted', 'paid'],