Support dotnet 8

[chenrui333]

On 14 Nov 2023, dotnet 8 went to the LTS status, it would be good to have it supported.

Trying to build against dotnet 8.0.0 with dafny 4.4.0, ran into some build failure as below:

/private/tmp/dafny-20240107-24453-99harp/dafny-4.4.0/Source/DafnyCore/DafnyCore.csproj(6,5): error MSB3073: The command "dotnet tool run coco "/private/tmp/dafny-20240107-24453-99harp/dafny-4.4.0/Source/DafnyCore/Dafny.atg" -namespace Microsoft.Dafny -frames "/private/tmp/dafny-20240107-24453-99harp/dafny-4.4.0/Source/DafnyCore/Coco"" exited with code 150.
    0 Warning(s)
    1 Error(s)

[atomb]

PR #5322 changes the build to use .NET 8. There are a couple of tests to fix before merging it, but I think those can be fixed pretty quickly.

[GGG-KILLER]

We've hit this issue while removing .NET 6 and 7 from nixpkgs as .NET 6 has hit EOL: NixOS/nixpkgs#326335