[IMP] base: merge activities of merged partners

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

Part-of: odoo/odoo#76159
Co-authored-by: Thibault Delavallee <tde@odoo.com>