Update dependencies and target frameworks

[Swimburger]

Update dependencies and TFMs:

• Use latest version of .NET (9) for running tests
• Build for .NET 7, 8, and 9 TFMs, drop .NET 6 support as it is out of support from Microsoft.
• Update dependencies
• Add PolySharp to use latest C# version across .NET Core and .NET Framework projects
• Enable nullable lang feature

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[Swimburger]

@dprothero Can you review this?

[dprothero]

@dprothero Can you review this?

@Swimburger looks like the same issue with the test reporting

[Swimburger]

CI passing!

[AJLange]

Thank you so much for supporting this! Looks like everything is passing and good to go now!