There are several other posts about this now and they all point to enabling TLS 1.2. Anything less is unsafe.
You can do this in .NET 3.5 with a patch.
You can do this in .NET 4.0 and 4.5 with a single line of code
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12; // .NET 4.5 ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072; // .NET 4.0
In .NET 4.6, it automatically uses TLS 1.2.
See here for more details: .NET support for TLS