We cache whether a project has an External Issue Tracker integration
enabled in the `has_external_issue_tracker` column on `projects`. It
will be `TRUE` if the project has any integration (a record in
`services`) with a category of "issue_tracker" enabled.

This was previously maintained with application code.

This cache is easy to fall out of consistency when we fail to maintain
it during bulk operations (including PostgreSQL cascading deletes
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/48163/) as
mentioned in:

- https://gitlab.com/gitlab-org/gitlab/-/issues/273574#note_457006686
- https://gitlab.com/gitlab-org/gitlab/-/issues/289798

As Ecosystem is increasingly changing integration data using bulk
operations this has been changed to be maintained through a PostgreSQL
trigger.

https://gitlab.com/gitlab-org/gitlab/-/issues/290715
https://gitlab.com/gitlab-org/gitlab/-/issues/289798