Upgrade to .NET 6 (#479)

* 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

.config/dotnet-tools.json

@@ -3,7 +3,7 @@
   "isRoot": true,
   "tools": {
     "cake.tool": {
-      "version": "1.1.0",
+      "version": "2.2.0",
       "commands": [
         "dotnet-cake"
       ]