Expose iteration property for issues API

What does this MR do and why?

Resolves #332377 (closed).

How to set up and validate locally

1. Create an iteration (visit Iteration under Issues via the left menu)

   Note that iterations are group-scoped objects.

2. Assign the iteration to an issue.

3. Test out the API endpoints:

   Single issue: http://gdk.test:3000/api/v4/issues/<issue id>

   Project issues: http://gdk.test:3000/api/v4/projects/<project id>/issues

   Group issues: http://gdk.test:3000/api/v4/group/<group id>/issues

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.