Add assignees widget for incident issue types
What does this MR do and why?
Adds migration to add the Assignee widget definition for Incident issue/work item types, in line with other underlying work item types.
Follows the model of https://gitlab.com/gitlab-org/gitlab/blob/master/db/migrate/20230228142350_add_notifications_work_item_widget.rb
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.