can not use $azfw.PrivateRange

## Description
When I used below command in order to add private address range on Azure Firewall, the operation was failed.

m> $azfw = Get-AzFirewall -Name AZFW01 -ResourceGroupName NVA-rg1
PS C:\Users\ishiyam> $azfw.PrivateRange = @("255.255.255.255/32")
Exception setting "PrivateRange": "'255.255.255.255/32' is not a valid private range ip address, bits not covered by subnet mask should be all 0"
At line:1 char:1
+ $azfw.PrivateRange = @("255.255.255.255/32")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], SetValueInvocationException
+ FullyQualifiedErrorId : ExceptionWhenSetting

When I used other range, the command was successful.

PS C:\Users\ishiyam> $azfw.PrivateRange = @("200.200.200.200/32")
PS C:\Users\ishiyam>

I also confirmed that I could add the range "255.255.255.255/32" from Portal so this is bug of powershell.


I think this source is related to this issue.
https://github.com/Azure/azure-powershell/blob/master/src/Network/Network/Models/AzureFirewall/PSAzureFirewall.cs

## Environment data
PS C:\Users\ishiyam> Get-InstalledModule -Name Az -AllVersions | select Name,Version

Name Version
---- -------
Az 5.6.0

Reproduce the assignment with Get-AzFirewall and the 255.255.255.255/32 range, then inspect src/Network/Network/Models/AzureFirewall/PSAzureFirewall.cs, which the report identifies as related. Compare it with the successful 200.200.200.200/32 assignment and the Azure Portal behavior. Done means the PowerShell object accepts the reported range consistently with the portal.

Written by the indexing model from the issue text.