Update New-CsAutoAttendant.md

[CLYVR]

Updating to indicate the UserNameExtension is in PowerShell only and that changes in TAC will remove this setting.

[learn-build-service-prod]

Learn Build status updates of commit 1071952:

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[CLYVR]

@mkbond007 Please and thanks!

[learn-build-service-prod]

Learn Build status updates of commit 594bc20:

✅ Validation status: passed

File	Status	Preview URL	Details
teams/teams-ps/teams/New-CsAutoAttendant.md	✅Succeeded	View (teams-ps)

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[learn-build-service-prod]

Learn Build status updates of commit 00cd509:

✅ Validation status: passed

File	Status	Preview URL	Details
teams/teams-ps/teams/New-CsAutoAttendant.md	✅Succeeded	View (teams-ps)

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[learn-build-service-prod]

Learn Build status updates of commit 2b2269e:

✅ Validation status: passed

File	Status	Preview URL	Details
teams/teams-ps/teams/New-CsAutoAttendant.md	✅Succeeded	View (teams-ps)

For more details, please refer to the build report.

For any questions, please:

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