Update versions for AzureRM.Storage 6.0.0-preview release

[markcowl]

Description

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
  • the cmdlet markdown files were generated using the platyPS module

[markcowl]

signing: https://azuresdkci.westus2.cloudapp.azure.com/view/PowerShell/job/ps-sign/1356/

[blueww]

@markcowl
The signed build has pass validation, so we can sign off it.
Please help to release it on 9/24.