Usagesummary.ps1 does not properly handle [DateTime] variants for StartTime/EndTime

Issue was reported on stamp version 2005, however the report generates on 2002 stamp properly with the same inputs.

Ask: Validate Usagesummary.ps1 on stamp versions > 2002. I have not tested this yet.

https://github.com/Azure/AzureStack-Tools/blob/master/Usage/Usagesummary.ps1

```
PS C:\Users\vmadmin\Documents> .\usagesummary.ps1
cmdlet usagesummary.ps1 at command pipeline position 1
Supply values for the following parameters:
StartTime: 2021-05-20
EndTime: 2021-06-01
Get-AzsSubscriberUsage : The reported date time '2021-05-20T00:00:00.0000000 00:00' is not in ISO 8601 format(YYYY-MM-DDTHH:mm:ss.sssZ).
At C:\Users\vmadmin\Documents\usagesummary.ps1:129 char:1
+ $result = Get-AzsSubscriberUsage -ReportedStartTime ("{0:yyyy-MM-ddT0 ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: ({ SubscriptionI...ationToken = }:<>f__AnonymousType0`6) [Get-AzsSubscriberUsage_List], UndeclaredResponseException
+ FullyQualifiedErrorId : BadRequest,Microsoft.Azure.PowerShell.Cmdlets.CommerceAdmin.Cmdlets.GetAzsSubscriberUsage_List
```

Switching line 129 to this and it works:
```
$result = Get-AzsSubscriberUsage -ReportedStartTime $StartTime -ReportedEndTime $EndTime -AggregationGranularity $Granularity
```