[FIX] project: add follower as project user

- Create a user A with access rights to Project set as 'User'
- As user A, add a follower to a task

An AccessError is raised.

It happens because `allowed_user_ids` has the group
`project.group_project_manager`.

Since a Project User is allowed to set followers on a task, it is
legitimate to set the appropriate portal user as allowed.

opw-2369674

closes odoo/odoo#60702

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/project/models/project.py

@@ -997,7 +997,7 @@ class Task(models.Model):
         if partner_ids:
             new_allowed_users = self.env['res.partner'].browse(partner_ids).user_ids.filtered('share')
             tasks = self.filtered(lambda task: task.project_id.privacy_visibility == 'portal')
-            tasks.write({'allowed_user_ids': [(4, user.id) for user in new_allowed_users]})
+            tasks.sudo().write({'allowed_user_ids': [(4, user.id) for user in new_allowed_users]})
         return res
 
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