Update to .NET 6, use project-based FAKE build instead of script

[kMutagene]

Hi there, In this PR i took a naïve go at upating the project to .NET 6:

• updated the fornax tool to target .NET 6
• migrated the FAKE build pipeline to a runnable project in the repo root. The reason for that was that i could not for the life of me make the build script work with updated dependencies via paket (some error about not finding Microsoft.Build.Framework). This project-based build is in my experience more stable than scripts. the SAFE stack for example uses the same approach.

I made sure that the tool works locally using the result of dotnet publish as well as the nuget package from the Pack target, i was however not sure about this line:

https://github.com/ionide/Fornax/blob/4187a3818807d65fc05365bb9aa34f2cb87a78c3/src/Fornax/FSIRefs.fs#L133

as the net5.0 tool would not run with 3.1 sdk anyways, but to mirror this pattern i left both 3.1 and 5.0 lines in that file intact.

[kMutagene]

can i further help to get this merged?