Deprecate omniauth-cas3 for removal in 16.0

We have decided to remove omniauth_cas3 in %16.0. This issue is to track submitting a deprecation post.

Why?

1. In our usage, cas3 is last by a large margin.
2. As part of our upgrade to OmniAuth, which is very important for security, we discovered that cas3 does not have a compatible upstream release, and the maintainer is not responding to requests.