[Identity] Update AzurePowerShellCredential script

[pvaneck]

• Only add the -AsSecureString argument when available and needed. This argument is no longer needed for Az.Accounts versions 5.0.0 and above.
• Update secure string parsing logic to allow it to work if a user is using Windows PowerShell instead of PowerShell 7+. Alternative parsing logic pulled from https://learn.microsoft.com/en-us/powershell/azure/migrate-az-14.0.0?view=azps-14.1.0#azaccounts.

Closes: #41508

[Copilot]

Pull Request Overview

This PR refines the AzurePowerShellCredential helper by restricting the -AsSecureString flag to Az.Accounts versions 2.17.0–<5.0.0 and adds a fallback path for parsing SecureString tokens on Windows PowerShell (<7). It also updates the changelog to document this bug fix.

• Limit -AsSecureString usage to the intended Az.Accounts version range.
• Introduce SecureString-to-plaintext conversion logic for PSVersion < 7.
• Add a changelog entry for the Windows PowerShell compatibility fix.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
sdk/identity/azure-identity/azure/identity/_credentials/azure_powershell.py	Adjusted version condition and added branch for SecureString parsing
sdk/identity/azure-identity/CHANGELOG.md	Inserted bug fix entry for Windows PowerShell SecureString support

Comments suppressed due to low confidence (2)

sdk/identity/azure-identity/azure/identity/_credentials/azure_powershell.py:51

• This new branch for Windows PowerShell (<7) parsing logic should be covered by automated tests to ensure both PS 7+ and Windows PowerShell behaviors are validated.

    if ($PSVersionTable.PSVersion.Major -lt 7) {{

sdk/identity/azure-identity/CHANGELOG.md:11

• Please confirm that #41675 matches this PR number, and consider also linking the original issue (Update AzurePowerShellCredential for breaking change in Get-AzAccessToken #41508) in the changelog entry for clearer tracking.

- Fixed an issue with `AzurePowerShellCredential` not working correctly for users still using older versions of PowerShell (e.g., Windows PowerShell 5.1) where `-AsPlainText` is not supported in the `ConvertFrom-SecureString` cmdlet.  ([#41675](https://github.com/Azure/azure-sdk-for-python/pull/41675))