The id of `customIpPrefix` with customIpPrefix name in uppercase returned by 'Public IP Prefixes - Create Or Update' API

The id of `customIpPrefix` with customIpPrefix name in uppercase returned by '[Public IP Prefixes - Create Or Update' API](https://github.com/Azure/azure-rest-api-specs/blob/c09b26d4aaa40fb37380aae01b7019b75cff3e34/specification/network/resource-manager/Microsoft.Network/stable/2021-03-01/publicIpPrefix.json#L152) :

Repro steps:

Create a publicIpPrefix resource (specify `customIPPrefix` property)
The id of "customIpPrefix" in the following request body:
```
"customIPPrefix": {
"id": /subscriptions/XXX/resourceGroups/test/providers/Microsoft.Network/customIpPrefixes/testpip01"
}
```

The id of "customIPPrefix" returned in the following response body:

```
"customIpPrefix": {
"id": "/subscriptions/XXX/resourceGroups/test/providers/Microsoft.Network/customIPPrefixes/testpip01"
}
```

Expected:
The id of `customIpPrefix` with custom Ip Prefix name should be `customIpPrefixes`.

Actual:
The id of `customIpPrefix` with custom Ip Prefix name is `customIPPrefixes`.

Inspect specification/network/resource-manager/Microsoft.Network/stable/2021-03-01/publicIpPrefix.json around the Public IP Prefixes Create Or Update operation linked in the issue. Compare the customIPPrefix request and response schemas with the reported casing, then check the repository’s validation or example conventions. Done means the API specification consistently represents the expected customIpPrefixes identifier casing and its checks pass.

Written by the indexing model from the issue text.