fix: Accounting for upcoming breaking change of Get-AzAccessToken

[AlexanderSehr]

Description

This PR is accounting for an upcoming breaking change that changes the return value of Get-AzAccessToken:

WARNING: Upcoming breaking changes in the cmdlet 'Get-AzAccessToken' :
The Token property of the output type will be changed from String to SecureString. Add the [-AsSecureString] switch to avoid the impact of this upcoming breaking change.
- The change is expected to take effect in Az version : '13.0.0'
- The change is expected to take effect in Az.Accounts version : '4.0.0'
Note : Go to https://aka.ms/azps-changewarnings for steps to suppress this breaking change warning, and other information on breaking changes in Azure PowerShell.

We effectively, proactively already switch to a secureString which will eventually be the default.

Pipeline Reference

Pipeline

Type of Change

• Update to CI Environment or utilities (Non-module affecting changes)
• Azure Verified Module updates:
  • Bugfix containing backwards-compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in version.json:
    • Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • The bug was found by the module author, and no one has opened an issue to report it yet.
  • Feature update backwards compatible feature updates, and I have bumped the MINOR version in version.json.
  • Breaking changes and I have bumped the MAJOR version in version.json.
  • Update to documentation

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