Add information about .NET

[krompaco]

Would be nice to add dotnet and which versions are included to this documentation file: https://github.com/netlify/build-image/blob/focal/included_software.md

Just the output of dotnet --list-sdks would do I suppose.

I'm successfully running builds with 6.0 so I can confirm it is in there and works perfectly 😅

Thanks for all your great work!