sni_hostname renamed to server_name

[cpu]

The upstream Rustls project changed the ServerConnection's sni_hostname function to be named server_name. See rustls/rustls@9b8fa40.

I think it makes sense to duplicate this change in the rustls-ffi API, changing rustls_server_connection_get_sni_hostname to be rustls_server_connection_get_server_name. I think its clearer to match upstream and it would also be consistent with existing fns like rustls_accepted_server_name.

Something like this: cpu@c68e826

[cpu]

Resolved in the rustls-0.21.0 branch.