New-AzApplicationGateway parameter sets don't allow using Identity and FirewallPolicy together

### Description

See title. Seems related to [New-AzApplicationGateway parameter sets don't allow using UserAssignedIdentityId and FirewallPolicyId together](https://github.com/Azure/azure-powershell/issues/24335)

Now that I'm getting messages that tell me I need to convert to using policies in App Gateway_WAF2, I'm kind of stuck.

### Issue script & Debug output

```PowerShell
New-AzApplicationGateway : Parameter set cannot be resolved using the specified named parameters.
At line:1 char:10
```

### Environment data

```PowerShell
Name Value
---- -----
PSVersion 5.1.19041.4170
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.19041.4170
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
```

### Module versions

```PowerShell
ModuleType Version Name
---------- ------- ----
Script 2.16.0 Az.Accounts
Script 4.0.2 Az.ApiManagement
Script 5.2.1 Az.KeyVault
Script 1.2.0 Az.ManagedServiceIdentity
Script 7.4.0 Az.Network
Script 1.0.4 Az.PrivateDns
Script 6.16.0 Az.Resources
Script 3.2.0 Az.Websites
```

### Error output

_No response_

Start by locating the New-AzApplicationGateway parameter sets in the Azure PowerShell Network module and reproduce the failure with Identity and FirewallPolicy together. Compare the related issue about UserAssignedIdentityId and FirewallPolicyId; done means the cmdlet accepts both options and the relevant parameter-set behavior is covered by tests.

Written by the indexing model from the issue text.