Add deprecation message to AddAzureASAccount cmdlet.

[gapreste]

Description

Add deprecation message to Add-AzAnalysisServicesAccount cmdlet, since it will be removed in the next breaking change update.

Checklist

• I have read the Submitting Changes section of CONTRIBUTING.md
• The title of the PR is clear and informative
• The appropriate change log has been updated
• The PR does not introduce breaking changes
• If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • the changes have gone through a cmdlet design review (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module