Update Microsoft.Rest.ClientRuntime to 2.3.20

[cormacpayne]

Description

Fix for #9513

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 (Microsoft internal partners only)
  • the cmdlet markdown files were generated using the platyPS module

[cormacpayne]

Sign job: https://azuresdkci.westus2.cloudapp.azure.com/job/psnetsign-esrp/431/

[cormacpayne]

Corresponding common code changes: Azure/azure-powershell-common#145

Verified the functionality passes smoke tests on both Windows PowerShell and PowerShell Core.