Enable Tls13 support by requiring Windows .NET Framework 4.8 or higher

[ddemeyer]

Problem Statement
Currently the HTTPS handshake is limited to
ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12. Adding SecurityProtocolType.Tls13 requires to raise platform support from .NET Framework 4.5 and up to .NET Framework 4.8 (and up).

See framework versions on https://docs.microsoft.com/en-us/dotnet/api/system.net.securityprotocoltype?view=netframework-4.8

Solution

• Raise the major version of the module, as this is a breaking change. And raise .NET Framework to 4.8 and extend the flags in ServicePointManagerHelper.cs.
• Leave as-is, postpone the change and look into the future for an OpenAPI-based but also .NET Core 3.1+ based alternative which is also a breaking change.