Invalid Policy Expression Sample

Policy example [Get OAuth2 access token from AAD using client id and certificate using key vault manage identity](https://github.com/Azure/api-management-policy-snippets/blob/f647c32a7eaf392a9935e2fef9bb1d13a399207d/examples/Get%20OAuth2%20access%20token%20from%20AAD%20using%20client%20id%20and%20certificate%20using%20key%20vault%20manage%20identity.xml#L65) uses RSACng for its signing operations. However, per the [APIM Policy Allowed Types list](https://learn.microsoft.com/en-us/azure/api-management/api-management-policy-expressions#CLRTypes) this is not an allowed type and attempting to use this type in an APIM policy results in the error message `Usage of type 'System.Security.Cryptography.RSACng' is not supported within expressions`

Start with the linked XML example at line 65 and compare its signing type with the APIM Policy Allowed Types list. Check how the sample behaves when evaluated as an APIM policy; done means the example no longer relies on the unsupported type and the reported error is resolved.

Written by the indexing model from the issue text.