[FIX] mass_mailing: open duplicate in edit mode

Since the new views, the flag `initial_view` on an action has been removed.

There were in fact multiple ways to do the same thing (open a form view in edit
mode directly with an action) (the supported option is `form_view_initial_mode`
in the context) ; the options have been cleaned up a little bit.

This was the only remaining recurrence.

Closes #22353

addons/mass_mailing/models/mass_mailing.py

@@ -520,14 +520,15 @@ class MassMailing(models.Model):
         self.ensure_one()
         mass_mailing_copy = self.copy()
         if mass_mailing_copy:
+            context = dict(self.env.context)
+            context['form_view_initial_mode'] = 'edit'
             return {
                 'type': 'ir.actions.act_window',
                 'view_type': 'form',
                 'view_mode': 'form',
                 'res_model': 'mail.mass_mailing',
                 'res_id': mass_mailing_copy.id,
-                'context': self.env.context,
-                'flags': {'initial_mode': 'edit'},
+                'context': context,
             }
         return False