https://blog.****.net/sun49842566/article/details/82802297
net 4.0 设置： ServicePointManager.SecurityProtocol = (SecurityProtocolType)3072;
//.net 4.5 设置： ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls1.2;
附上官方说明
https://docs.microsoft.com/zh-cn/dotnet/api/system.net.securityprotocoltype?view=netframework-4.7.2