[FW][FIX][14.0][accounting][l10n_lu] Notify account manager one by one #3119912

[fw-bot]

Description of the issue/feature this PR addresses:
Traceback during update of Odoo accounting.

  File "/home/francois/.virtualenvs/odoo-abitare14/lib/python3.8/site-packages/odoo/addons/l10n_lu/migrations/2.1/post-migrate_update_taxes.py", line 6, in migrate
    update_taxes_from_templates(cr, 'l10n_lu.lu_2011_chart_1')
  File "/home/francois/.virtualenvs/odoo-abitare14/lib/python3.8/site-packages/odoo/addons/account/models/chart_template.py", line 156, in update_taxes_from_templates
    _notify_accountant_managers(outdated_taxes)
  File "/home/francois/.virtualenvs/odoo-abitare14/lib/python3.8/site-packages/odoo/addons/account/models/chart_template.py", line 131, in _notify_accountant_managers
    partner_managers_ids.message_post(
  File "/home/francois/.virtualenvs/odoo-abitare14/lib/python3.8/site-packages/odoo/addons/rating/models/mail_thread.py", line 14, in message_post
    message = super(MailThread, self).message_post(**kwargs)
  File "/home/francois/.virtualenvs/odoo-abitare14/lib/python3.8/site-packages/odoo/addons/mail/models/mail_thread.py", line 1823, in message_post
    self.ensure_one()  # should always be posted on a record, use message_notify if no record
  File "/home/francois/.virtualenvs/odoo-abitare14/lib/python3.8/site-packages/odoo/models.py", line 5028, in ensure_one
    raise ValueError("Expected singleton: %s" % self)

Current behavior before PR:
NA

Desired behavior after PR is merged:
The update should work.

To reproduce, you just need to have a previous version of l10n_lu module and more than 1 account manager.
Then update your Odoo and you'll have the traceback.

Ticket #3119912

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Forward-Port-Of: #109062

[robodoo]

Pull request status dashboard.

[fw-bot]

This PR targets saas-15.3 and is part of the forward-port chain. Further PRs will be created up to master.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[fw-bot]

@acsonefho @william-andre the next pull request (#109176) is in conflict. You can merge the chain up to here by saying

@fw-bot r+

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[william-andre]

@fw-bot r+