Add iterations cadences to Gitlab Migration

What does this MR do and why?

This MR adds iterations cadences (which include iterations within them) to GitLab Group Migration (https://docs.gitlab.com/ee/user/group/import/).

Our existing solution migrations iterations (using GraphQL), without cadences. This MR starts to include cadences as well. The previous approach wasn't exactly correct because all iterations were put into 'default cadence', losing the connection with iterations cadence. This MR updates the way we migrate iterations and adds cadences.

Docs are updated in the separate MR !96883 (merged)

Mentions #291983 (closed)

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