API don't return needed information to grant SPN permissions in Wiki Create Landingzones

**Describe the bug**

```powershell
# Request billing accounts that the identity has access to
$listOperations = @{
Uri = "https://management.azure.com/providers/Microsoft.Billing/billingaccounts?api-version=2020-05-01"
Headers = @{
Authorization = "Bearer $($token.Token)"
'Content-Type' = 'application/json'
}
Method = 'GET'
}
$listBillingAccount = Invoke-RestMethod @listOperations

# List billing accounts
$listBillingAccount | ConvertTo-Json -Depth 100

# Select first billing account and the corresponding enrollment account
$billingAccount = $listBillingAccount.value[0].id
$enrollmentAccountId = $listBillingAccount.value[0].properties.enrollmentAccounts[0].id
```
Since enrollmentAccount isn't returned in the api call $enrollmentAccountId throw a null exception when trying to assign to a null array.

Hence the following steps in the guide don't work.

I've tried to fix it with bringing more by using the api documentation but it won't return more information
```'https://management.azure.com/providers/Microsoft.Billing/billingAccounts?api-version=2020-05-01&$expand=enrollmentDetails,departments,enrollmentAccounts'```

If you use the **2019-10-01-preview** endpoint all works as expected.

**Steps to reproduce**

1. Follow the guide

Start with the Wiki Create Landingzones guide and reproduce the billing account request using the 2020-05-01 endpoint. Compare its response with the 2019-10-01-preview endpoint and update the guide so the enrollment account information can be obtained; verify the documented steps complete without a null exception.

Written by the indexing model from the issue text.