Add Targetting .Net Framework 4.7.2 and remove Bouncy Castle dependancy

[ewwink]

.Net Framework 4.7.2 and .Net Standard 2.1 has class CertificateRequest for generating certificate
You can see example code for creating/chaining certificate here

[justcoding121]

We cannot get rid of bouncy castle because, we also have to support non-windows machines.

There is already an option to not use BouncyCastle for Windows.

https://github.com/justcoding121/Titanium-Web-Proxy/blob/develop/src/Titanium.Web.Proxy/Certificates/CertificateManager.cs#L19

https://github.com/justcoding121/Titanium-Web-Proxy/blob/develop/src/Titanium.Web.Proxy/Certificates/WinCertificateMaker.cs

It had a bug in the past, not sure if it is already fixed. If someone could replace that with thus CertificateRequest may be @honfika can review when he gets a chance