* Upgrade to .NET 6
.NET 5 has gone EOL, so move to a supported version.
* Upgrade Docker container as well
* Update docs for .NET 6
* Re-add hack for System.IO.Pipelines
* Bump dependency versions
* Update the deps of the other projects too
* Only declare TargetFramework once
* Dockerfile: also copy over Directory.Build.props
* Cake: pick up target framework automatically
* Upgrade Cake as well
miniduikboot