Update cross-tenant-mailbox-migration.md

[msbemba]

trafficstars

the note about the Domain Sharing for email private preview has been removed.

reference https://dev.azure.com/Supportability/M365%20Release%20Announcements/_wiki/wikis/M365-Product-Updates.wiki/525287/Domain-Sharing-for-Email-Preview

Fixes https://github.com/MicrosoftDocs/microsoft-365-docs-pr/issues/23573

[msbemba]

@JohanFreelancer9 Please copy review

[learn-build-service-prod[bot]]

Learn Build status updates of commit fcc6882:

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File	Status	Preview URL	Details
microsoft-365/enterprise/cross-tenant-mailbox-migration.md	:white_check_mark:Succeeded

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[yogkumgit]

@kelleyvice-msft Please merge this PR. Thanks

[yogkumgit]

@kelleyvice-msft Gentle reminder.