Deprecate get_mtls_endpoint_and_cert_source method

[ohmayr]

The method get_mtls_endpoint_and_cert_source is no longer used in the client i.e. see the following: https://github.com/googleapis/gapic-generator-python/pull/1873

This method can be deprecated after the PR https://github.com/googleapis/gapic-generator-python/pull/1868 is merged i.e. users can alternatively access the api endpoint via the api_endpoint property.

Update the docstring and add a deprecation warning to the get_mtls_endpoint_and_cert_source method.