Set-AzAksCluster can not disable LoadBalancer managed outbound ip when setting LoadBalancer outbound ip prefix

### Description

For AKS cluster, we have a scenario to disable the load balancer managed outbound IPs and only use the public IP prefixes (those public IP prefixes are from our BYOIP). However, Set-AzAksCluster could not disable the load balancer managed outbound IPs.

When we attempted to switch from the load balancer managed outbound IPs to the public IP prefixes, the cmdlet (Set-AzAksCluster) does not allow to set (-LoadBalancerManagedOutboundIpCount) to 0 (or $null) when updating (-LoadBalancerOutboundIpPrefix). If we don't provide a value to (-LoadBalancerManagedOutboundIpCount), Set-AzAksCluster will keep both the load balancer managed IPs and the public IP prefixes in the load balancer outbound NAT rule.

This is the script I am using. It doesn't remove the load balancer managed outbound IPs from the load balancer outbound NAT rule.
> $aksCluster = Get-AzAksCluster -ResourceGroupName test-aks-cluster
> $aksCluster | Set-AzAksCluster -LoadBalancerOutboundIpPrefix "/subscriptions/\/resourceGroups/MC_test-aks-cluster_test-aks-cluster_eastus/providers/Microsoft.Network/publicIPPrefixes/test-egress"

At the same time, if I use the 'azure cli' for the same thing, the command below will remove the load balancer managed outbound IPs from the load balancer outbound NAT rule.
> az aks update --name test-aks-cluster2 --resource-group test-aks-cluster2 --load-balancer-outbound-ip-prefixes "/subscriptions//resourceGroups/MC_test-aks-cluster_test-aks-cluster_eastus/providers/Microsoft.Network/publicIPPrefixes/test-egress"

Roughly went through the code of Set-AzAksCluster, the load balancer managed IP is not set to null when setting the public IP prefixes. Probably we should fix it.


### Issue script & Debug output

```PowerShell
N/A
```

### Environment data

```PowerShell
PS C:\Users\figo> $PSVersionTable

Name Value
---- -----
PSVersion 5.1.22621.3880
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.22621.3880
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
```

### Module versions

```PowerShell
PS C:\Users\figo> get-module az.aks

ModuleType Version Name ExportedCommands
---------- ------- ---- ----------------
Script 6.0.3 Az.Aks {Disable-AzAksAddOn, Enable-AzAksAddOn, Get-AzAksCluster, Get-AzAksNodePool...}
```

### Error output

_No response_