Updating datamigration api to stable 06 30 2021

[temandr]

All SDK Contribution checklist:

Swagger pull request: Azure/azure-rest-api-specs#15259

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Note: Only applies to Microsoft.Azure.Management.[RP] or Azure.ResourceManager.[RP]

• Include updated management metadata.
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Old outstanding PR cleanup

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If PRs (including draft) has been out for more than 60 days and there are no responses from our query or followups, they will be closed to maintain a concise list for our reviewers.

[ArcturusZhang]

Basically LGTM, only one minor comment