Added Net6 in the conditional to take the reference System.Drawing.Co…

[luisfbn]

Added Net6 in the conditional to take the reference System.Drawing.Common. Net35 was removed due to System.Drawing.Bitmap type does not exist on it.

Summary

This PR fixes/implements the following bugs/features:

• [ ] https://github.com/codebude/QRCoder/issues/382

What existing problem does the pull request solve?**

Creating a .Net 6 Project and try to use QRCode type on it. Visual Studio can't compile due to: The type or namespace name 'QRCode' does not exist in the namespace 'QRCoder'

Closing issues

closes #382