Update CSharp projects to .NET 6 in sample k.direct-line-token

Is your feature request related to a problem?

Yes, the current BotFramework-WebChat samples for C# in k.direct-line-token are using .NET Core 3.1, which has reached its end of life. This poses potential security and compatibility risks for developers who are looking to use these samples in their projects.

Describe the suggestion or request in detail

I suggest updating the BotFramework-WebChat samples, specifically the csharp sample for direct-line-token, to use a more recent and supported version of .NET Core or .NET 5/6. This will ensure that developers are working with a version that receives security updates and is in line with current best practices.

Describe alternatives you have considered

1. Developers could manually update the samples to a newer version of .NET Core or .NET 5/6, but this might lead to potential issues or incompatibilities.
2. Another alternative is to provide multiple versions of the samples for different .NET versions, but this might increase the maintenance overhead.

Additional context

.NET Core 3.1 reaching its end of life means it won't receive further security updates or bug fixes. By updating the samples to a supported version, it ensures that developers have a secure and stable starting point for their projects.