Trusted launch default and VMSS flex default conflict | az vmss create

### Describe the bug

Running command `az vmss create` generates below error as VMSS flex does not supports SystemAssigned identity. This could be due to implementation of Trusted launch as default and VMSS flex as default.

Command: **az vmss create -n minvmss02 -g $rgName -l $loc**
Error: **"message":"Required parameter 'platformFaultDomainCount' is missing (null)."**

Command: **az vmss create --name vmssname --resource-group rgname --image imagealias --vm-sku vmsize**
Error: **[{"code":"InvalidParameter","target":"identity","message":"The value 'SystemAssigned' of parameter 'identity' is not allowed. Allowed values are: UserAssigned, None."}]}}**

### Related command

`az vmss create`

### Errors

{"status":"Failed","error":{"code":"DeploymentFailed","target":"/subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroups/cli1111rg/providers/Microsoft.Resources/deployments/vmss_deploy_ABoEgqebU41wMuCMFZTLjEEmovfylStS","message":"At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/arm-deployment-operations for usage details.","details":[{"code":"InvalidParameter","target":"platformFaultDomainCount","message":"Required parameter 'platformFaultDomainCount' is missing (null)."}]}}

{"status":"Failed","error":{"code":"DeploymentFailed","target":"/subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroups/cli1111rg/providers/Microsoft.Resources/deployments/vmss_deploy_5VCbrHJ2JsnJMgYaeGFMg6xRPQ6KHoTO","message":"At least one resource deployment operation failed. Please list deployment operations for details. Please see https://aka.ms/arm-deployment-operations for usage details.","details":[{"code":"InvalidParameter","target":"identity","message":"The value 'SystemAssigned' of parameter 'identity' is not allowed. Allowed values are: UserAssigned, None."}]}}

### Issue script & Debug output

Command: **az vmss create -n minvmss02 -g $rgName -l $loc --vm-sku Standard_D2s_v3 --image Ubuntu2204 --debug******

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### Expected behavior

VMSS flex is deployed with Gen2 OS image and Trusted launch security type.

### Environment Summary

azure-cli 2.54.0

core 2.54.0
telemetry 1.1.0

Dependencies:
msal 1.24.0b2
azure-mgmt-resource 23.1.0b2

Python location 'C:\Program Files\Microsoft SDKs\Azure\CLI2\python.exe'
Extensions directory 'C:\Users\ajkundna\.azure\cliextensions'

Python (Windows) 3.11.5 (tags/v3.11.5:cce6ba9, Aug 24 2023, 14:38:34) [MSC v.1936 64 bit (AMD64)]

Legal docs and information: aka.ms/AzureCliLegal

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### Additional context

_No response_