Allows AddTicketWorkItemType migration specs to be slow

What does this MR do and why?

spec/migrations/20230804053643_add_ticket_work_item_type_spec.rb#L55 was flagged as a slow spec in #467328

The spec file was introduces with Add Service Desk Ticket as a new Work Item type... (!127482 - merged) to add the ticket work item type.

After trying it out locally it turned out that all examples of the test file take quite long. So this MR adds allowed_to_be_slow: true to the describe block.

MR acceptance checklist

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Screenshots or screen recordings

⛔ backend only

How to set up and validate locally

Just run the spec file locally e.g.

bundle exec rspec spec/migrations/20230804053643_add_ticket_work_item_type_spec.rb

Related to #467328