.NET 6 support

[endeavour]

Any chance this could be updated to support .NET 6?

[rodrigo455]

@jeikabu I forked this project under
https://github.com/rodrigo455/NanomsgNG.NET

added the following:

• NNG v1.8.0
• TLS support with mbedtls libraries added to the runtime (v3.6.0)
• Fix Disposable Interface implementation
• Target .NET 8 and .NET 6 (@endeavour )

some changes were added based on the pull requests that were left open here.
feel free to review them and let me know if you would like it merged here.

a package has been published in nuget.org