Deprecation notice for sdn check REV-1174

Hi there,

We plan to deprecate and remove the ecommerce SDN check endpoint (ported into CybersourceSubmitAPIView instead). We are targeting removal after Monday, May 11, 2020.

Please read https://openedx.atlassian.net/browse/DEPR-90 for more information and to post any questions/comments. The proposed deadline for comments before acceptance is Monday, May 4, 2020.

Thanks, Diane Kaplan