[FIX] mail: remove readonly on recommended next activity

Fine-tuning of 463601a384.
It does not make sense to be able to write on mail.activity.type
using the related fields defined on mail.activity.

But this field is not a related one, and it is necessary to be writable to make
the 'recommended next activity' feature work at all.

opw 1974236

closes odoo/odoo#33325

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/mail/models/mail_activity.py

@@ -89,7 +89,7 @@ class MailActivity(models.Model):
         ('today', 'Today'),
         ('planned', 'Planned')], 'State',
         compute='_compute_state')
-    recommended_activity_type_id = fields.Many2one('mail.activity.type', string="Recommended Activity Type", readonly=True)
+    recommended_activity_type_id = fields.Many2one('mail.activity.type', string="Recommended Activity Type")
     previous_activity_type_id = fields.Many2one('mail.activity.type', string='Previous Activity Type', readonly=True)
     has_recommended_activities = fields.Boolean(
         'Next activities available',