Creation and use of `work_item_types` table
#### Steps
- [x] create `work_item_types` table - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55705
- [x] add `work_item_type_id` to `issues` table - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/55705
- [x] create initial base types in table - `issue`, `incident`, `test_case`, `requirement` - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67434
- [x] make sure new issues are created with both an `issue_type` and a `work_item_type_id` - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67638
- [x] populate each issue with corresponding `work_item_type_id` related to it's `issue_type` - https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71869
- [x] replace use of `issue_type` with `work_item_type_id` throughout code - https://gitlab.com/groups/gitlab-org/-/epics/10529
- [x] Drop `issues.issue_type` column - https://gitlab.com/gitlab-org/gitlab/-/issues/402699
- [ ] Deprecate issue_type as input in all APIs. Add work_item_type_id as input - https://gitlab.com/gitlab-org/gitlab/-/issues/403006
- [ ] Update docs - https://gitlab.com/gitlab-org/gitlab/-/issues/417670
Proposed schema changes
```plantuml
hide circle
hide empty members
entity issues {
used for WorkItem/Issue model
-- normal attributes --
+ id
+ project_id
+ milestone_id
+ etc...
---
+ issue_type
+ work_item_type_id
}
entity work_item_types {
+ id
+ type
+ namespace_id
+ title
+ description
+ description_html
+ cached_markdown_version
+ icon_name
-- has_many --
- work_items
}
entity work_item_type_enum {
* issue
* incident
* test_case
* requirement
}
issues::work_item_type_id <-- work_item_types::work_items
issues::issue_type -[dashed]- work_item_type_enum
work_item_types::type -[dashed]- work_item_type_enum
```
_Note: the `issue_type` on the `issues` table is mostly for the transition. If it has any performance value (such as reducing a query to the type table), then we might consider kepping it. Otherwise, the plan is to remove it and rely only on the `type` field in the `work_item_types` table._
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