When merging partners through the ``base.partner.merge.automatic.wizard``
wizard, messages and followers are merged. However activities are not
probably because its underlying model is "newer" then messages and followers.
This commits fixes that behavior.
Task-2641572
PR odoo#76159
Closes#71654
X-original-commit: 7e17678ec6ebbbe37b807a5d264cc5afd46bae1a
Part-of: odoo/odoo#77005
Co-authored-by: Thibault Delavallee <tde@odoo.com>