Add support for preview versions of .NET core

[kvedurmu]

What happened?

Could we look into adding support for preview versions of .NET Core in the buildpack? .NET Core 6 is currently available in preview state (https://dotnet.microsoft.com/download/dotnet/6.0) and will go GA in November 2021. It would be helpful to users if they could use the buildpack to target a preview version of .NET Core so that they have sufficient time to build/test their apps against latest .NET versions.

I'd expect this to be entirely opt-in and users should be able to opt-in by setting prerelease key in their global.json (https://docs.microsoft.com/en-us/dotnet/core/tools/global-json?tabs=netcore3x).

[fg-j]

@kvedurmu Transferring this to the language family because this will entail changes to the runtime, ASPNET, and SDK buildpacks.

[fg-j]

We are still investigating the maintenance overhead that would come with supporting preview versions of .NET. More indication of user demand would help signal that we should prioritize this!

[ForestEckhardt]

This could become much easier with the changes that we are making to the dependency management model that we use and could become even more obtainable if we change up our runtime and aspnet dependencies