AzureManagedControlPlane doesn't support managedNATGateway

[bogdanmicu-del]

/kind bug

What steps did you take and what happened:
Try to create an azure cluster using AzureManagedControlPlane and AzureManagedCluster and setting outboundType: managedNATGateway.

What did you expect to happen:
The Azure cluster should be created successfully.

Anything else you would like to add:
I can see this error reported on the AzureManagedControlPlane object:
{ "code": "InvalidParameter", "details": null, "message": "Outbound type is managedNATGateway but agent pool '' is using custom VNet, which is not allowed.", "subcode": "ManagedNATGatewayWithCustomVNetNotAllowed", "target": "properties.vnetSubnetID" }

A similar problem was reported over here.

Environment:

• cluster-api-provider-azure version: v1.17.2
• Kubernetes version: (use kubectl version):
  Client Version: v1.31.0 Kustomize Version: v5.4.2 Server Version: v1.30.7
• OS (e.g. from /etc/os-release): N\A

[paulapetri]

@dtzar - can you help us with a fix for this?

[willie-yao]

/assign @nawazkh