Support for .NET6 (November 2021)

[TimothyMothra]

Opening an issue to track supporting .NET 6.

Their milestone is listed for Nov 12: https://github.com/dotnet/runtime/milestone/69

Preview 6 was released on July 14th. There will be a Preview 7 and two RCs before the final release.

• https://devblogs.microsoft.com/dotnet/announcing-net-6-preview-6/

TODO (After GA Release):

• [x] Remove "preview" flags introduced in PR #2391
• [x] Upgrade <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="6.0.0-rc.2.21480.10" /> in IntegrationTests.Tests.csproj introduced in PR #2441
• [x] Remove "preview" from build definitions

[cijothomas]

We need to club this (https://github.com/microsoft/ApplicationInsights-dotnet/issues/2273) as well for the .NET 6 support, as .NET6 removes support for net452,net46.