Add Az 13 migration guide

[vidai-msft]

Description

Add migration guide.

Mandatory Checklist

• Please choose the target release of Azure PowerShell. (⚠️Target release is a different concept from API readiness. Please click below links for details.)

  • [x] General release
  • [ ] Public preview
  • [ ] Private preview
  • [ ] Engineering build
  • [ ] No need for a release

• [ ] Check this box to confirm: I have read the Submitting Changes section of CONTRIBUTING.md and reviewed the following information:

• SHOULD update ChangeLog.md file(s) appropriately

  • For SDK-based development mode, update src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md.
    • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header in the past tense.
  • For autorest-based development mode, include the changelog in the PR description.
  • Should not change ChangeLog.md if no new release is required, such as fixing test case only.

• SHOULD regenerate markdown help files if there is cmdlet API change. Instruction

• SHOULD have proper test coverage for changes in pull request.

• SHOULD NOT adjust version of module manually in pull request

[azure-client-tools-bot-prd[bot]]

️✔️Az.Accounts

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