Add destroyIteration mutation

What does this MR do?

Adds destroyIteration mutation to GQL

Relates to #292268 (closed)

Lack of Foreign key on iteration_id explanation

This is something every reviewer has asked, so it makes sense to clarify here. The iteration_id column does not have a foreign key as would be expected, and the reason for this is that the current implementation of boards allows values like -4 in the column to refer to the "current iteration". That's why we have to manually update all boards that held the ID of the destroyed iteration.

Database plans

Query 1

RAW SQL

UPDATE "boards" SET "iteration_id" = NULL WHERE "boards"."iteration_id" = 17394

Query Plans

https://console.postgres.ai/shared/38a02859-f546-4460-b5c9-918b1f963752

Query 2

RAW SQL

SELECT 1 AS one
FROM "sprints"
WHERE "sprints"."iterations_cadence_id" = 913
  AND (start_date > '2021-05-01 00:00:00')
LIMIT 1;

Query Plans

https://console.postgres.ai/shared/37598333-ddd9-4f6d-b594-baca0b901f2d

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Related to #292268 (closed)