`Microsoft.Web/sites` - `customDomainVerificationId` should be marked `readonly`

As per https://github.com/Azure/bicep/issues/784#issuecomment-933188709, we believe this is not a property that can be set via a PUT request. If this is the case, please mark it as `readonly`. Example here:
https://github.com/Azure/azure-rest-api-specs/blob/88e7838a09868a51de3894114355c75929847a46/specification/web/resource-manager/Microsoft.Web/stable/2020-06-01/CommonDefinitions.json#L1832-L1835

Start with the linked CommonDefinitions.json section for Microsoft.Web/sites and compare the customDomainVerificationId property with the PUT behavior referenced in Azure Bicep issue 784. Confirm whether the property is server-generated, then update its specification metadata to readonly and validate the affected REST specification.

Written by the indexing model from the issue text.