Azure API for gov - Enable diagnostic settings for management groups

### Description of the new feature

This works in commercial:
https://docs.microsoft.com/en-us/rest/api/monitor/management-group-diagnostic-settings/create-or-update

This is what I see from a USGov tenant:


{"code":"BadRequest","message":"Configuration 'managementgroupactivitylogdiagnosticsettingconfig' doesn't exist}"

### Proposed implementation details (optional)

_No response_

Start with the linked Azure Monitor REST API documentation for management-group diagnostic settings and compare it with the reported USGov error, "Configuration 'managementgroupactivitylogdiagnosticsettingconfig' doesn't exist." Locate the Azure PowerShell entry point for management-group diagnostic settings and verify the work against a USGov tenant; done means the commercial capability is available there without that error.

Written by the indexing model from the issue text.