Use iid instead of id when updating epic attributes

What does this MR do?

iid attribute is used when updating an epic in graphql API so we pass it together with other attributes to frontend with other epic initialization data.

This is a regression introduced in a recent change this milestone (not adding changelog to this one) - !22467 (merged) and !22872 (merged) - client used epic's id instead of iid when sending a graphql request.

Related to #31921 (closed) and #31915 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

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