Misc fixes

[lukebakken]

• Add solution items
• Use default LangVersion

[Gsantomaggio]

Do we want to support only .8 ?

[lukebakken]

https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core

I was having issues with tests running correctly with all three TFMs in the projects. I'll look into adding net6.0 and net7.0 back.

Note that if we wanted to support as many versions of .NET, we'd do what AmqpNet.Core does -
https://www.nuget.org/packages/AMQPNetLite.Core#supportedframeworks-body-tab

https://learn.microsoft.com/en-us/dotnet/standard/net-standard?tabs=net-standard-1-0

[lukebakken]

@Gsantomaggio I'm currently seeing how much of a pain supporting netstandard2.0 is, like we do with the AMQP 0.9.1 .NET client.

[Gsantomaggio]

@lukebakken I don't have any strong opinion on support versions >= net.8
I asked just because if we want to support only >= 8 we can remove https://github.com/rabbitmq/rabbitmq-amqp-dotnet-client/blob/main/Directory.Packages.props#L22 System.Diagnostics.DiagnosticSource" an use the new .NET 8 library for the metrics: https://www.mytechramblings.com/posts/getting-started-with-opentelemetry-metrics-and-dotnet-part-1/

[Gsantomaggio]

Thank you @lukebakken 😃