Update Get-AzKeyVaultCertificate.md - Get cert and save it as pfx section to work with PowerShell Core

[dawsonar802]

Description

The existing syntax does not seem to work in PowerShell Core. Using the proposed syntax works with both PowerShell Desktop and Core. When Calling the 'import' in this line:
$x509Cert.Import($secretByte, "", "Exportable,PersistKeySet") in PowerShell core, an error is received:
"Import" with "3" argument(s): "X509Certificate is immutable on this platform. Use the equivalent constructor instead."

Declaring the cert object like this works fine:
$x509Cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($secretByte, "", "Exportable,PersistKeySet")

The requested change is a combination of the existing MS Documentation and the above which was pulled from: https://gist.github.com/holgerjay/0db1f759f93cba268d671341b787472a

Requested Change to the example code:

$secretValueText = '';
$ssPtr = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($secret.SecretValue)
try {
$secretValueText = [System.Runtime.InteropServices.Marshal]::PtrToStringBSTR($ssPtr)
} finally {
[System.Runtime.InteropServices.Marshal]::ZeroFreeBSTR($ssPtr)
}
$secretByte = [Convert]::FromBase64String($secretValueText)
$x509Cert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($secretByte, "", "Exportable,PersistKeySet")
$type = [System.Security.Cryptography.X509Certificates.X509ContentType]::Pfx
$pfxFileByte = $x509Cert.Export($type, $password)

Checklist

• [x ] I have read the Submitting Changes section of CONTRIBUTING.md
• [x ] The title of the PR is clear and informative
• The appropriate ChangeLog.md file(s) has been updated:
  • For any service, the ChangeLog.md file can be found at src/{{SERVICE}}/{{SERVICE}}/ChangeLog.md
  • A snippet outlining the change(s) made in the PR should be written under the ## Upcoming Release header -- no new version header should be added
• [x ] The PR does not introduce breaking changes
• [ x] If applicable, the changes made in the PR have proper test coverage
• For public API changes to cmdlets:
  • a cmdlet design review was approved for the changes in this repository (Microsoft internal only)
    • {Please put the link here}
  • the markdown help files have been regenerated using the commands listed here

[ghost]

Thank you for your contribution dawsonar802! We will review the pull request and get back to you soon.

[msJinLei]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[isra-fel]

Thank you @dawsonar802 for your contribution! I verifed and the solution works on both Windows PowerShell and PowerShell 7.1

[isra-fel]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).