Migrate existing service desk issues to tickets

For both SaaS and Self-Managed customers, migrate all Service Desk Issues to the work item type Ticket.

1. Scheduled for 17.8.
2. Because Service Desk issues and tickets use the same underlying data structure, we don't need to migrate data from one table to the other.
3. We only need to change the work item type of all legacy Service Desk issues.
4. This will require a batched background migration that collects all legacy Service Desk issues (from support bot Users::Internal.support-bot and service_desk_reply_to NOT NULL) and changes the work item type to ticket.
5. For gitlab.com we can do this whenever we want to.
6. For Self-Managed we aim for the 17.8 release because this should be a required stop.
7. We can handle all post-migration operations after the required stop in 17.9 and following.

Success Criteria

• There should be no change in the UI
• Where Service Desk issues previously showed up, issues list, Service Desk list, boards, related issues, references, they continue to show up
• Tickets maintain all their metadata: assignee, labels, epic, etc