Task is deprecated.

[davidbritch]

Add deprecation notice.

[prmerger-automator[bot]]

@davidbritch : Thanks for your contribution! The author(s) have been notified to review your proposed change.

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

Learn Build status updates of commit 75e7fef:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
task-reference/xamarin-license-v1.md	:white_check_mark:Succeeded

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