[9.0][FIX] purchase: fix Unread Messages filter

In v9 the right filter to use is based upon the message_needaction
field coming from mail.thread . Indeed this gives records containing
messages demanding an action which is the right way to use the
needaction mechanism in 9, like done in all other addons inheriting
from mail.thread.

Closes #16769 .

addons/purchase/purchase_view.xml

@@ -295,7 +295,7 @@
                     <separator/>
                     <filter name="not_invoiced" string="Not Invoiced" domain="[('invoice_status','=', 'to invoice')]" help="Purchase orders that include lines not invoiced."/>
                     <filter name="invoiced" string="Invoiced" domain="[('invoice_status','=', 'invoiced')]" help="Purchase orders that include lines not invoiced."/>
-                    <filter string="New Mail" name="message_unread" domain="[('message_unread','=',True)]"/>
+                    <filter string="Unread Messages" name="message_needaction" domain="[('message_needaction','=',True)]"/>
                     <group expand="0" string="Group By">
                         <filter string="Vendor" domain="[]" context="{'group_by':'partner_id'}"/>
                         <filter string="Order Date" domain="[]" context="{'group_by':'date_order'}"/>