Add trigger to update issues correct_work_item_type_id

What does this MR do and why?

Before we introduced a trigger on insert or update but we didn't account for updates of issues records that don't change the work_item_type_id column. This new trigger should be executed for every update and set the correct value only if it wasn't updated before.

With this trigger, it should be safe to add a NOT VALID FK on issues.correct_work_item_type_id. This is a second attempt to add the FK as before it caused an incident when updating issue records gitlab-com/gl-infra/production#18692 (closed)

Timeline of migrations

• Add first trigger, and FK - !167510 (merged)
• enqeue the background migration - !167972 (merged)
• drop FK due to incident - !168741 (merged)

MR acceptance checklist.

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Related to #493974 (closed)