I think the template you're looking for can be found at
Administer -> CiviMail -> Message Templates, i.e, navigate to
civicrm/admin/messageTemplates?reset=1 and click on
System Workflow Messages.
The template name is
Cases - Send Copy of an Activity. Note that you don't need to enable CiviCase component to view this template. It is displayed by default and used to send emails to assignees(both case and normal activities).
Also, if you want to disable the sending of email to assignee for some activity types, there is a new configuration setting added at
The setting is only displayed when the above checkbox of
Notify Activity Assignee is enabled.