Update aks-nat-agic to supported version

### Template

[aks-nat-agic](https://github.com/Azure/azure-quickstart-templates/tree/master/demos/aks-nat-agic)

### Issue Details

### Repro steps
1. Simply deploy the template
2. Observe error message `PodIdentity addon is not allowed since feature 'Microsoft.ContainerService/EnablePodIdentityPreview' is not enabled.`
3. Read [Microsoft.ContainerService/EnablePodIdentityPreview](https://learn.microsoft.com/en-us/azure/aks/use-azure-ad-pod-identity) documentation
4. Observe the following note in the documentation

> We recommend you review [Azure AD workload identity](https://learn.microsoft.com/en-us/azure/aks/workload-identity-overview) (preview). This authentication method replaces pod-managed identity (preview), which integrates with the Kubernetes native capabilities to federate with any external identity providers on behalf of the application.

### Resolution

Update the aforementioned template to use [Use an Azure AD workload identity (preview)](https://learn.microsoft.com/en-us/azure/aks/workload-identity-overview) instead.

Inspect the aks-nat-agic template in demos/aks-nat-agic and reproduce the deployment error described in the issue. Read the linked Azure AD workload identity documentation, then update the template to use that authentication method instead of PodIdentity and verify that deployment no longer reports the disabled preview feature.

Written by the indexing model from the issue text.