(build) Install all the .NET's

Not immediately clear what isn't working when running kudusync.exe, but
let's try installing all the .NET's, to see if it doesn't start working.

Author: gep13

.github/workflows/build.yml

@@ -48,7 +48,9 @@ jobs:
         with:
           dotnet-version: |
             2.1.x
+            3.1.x
             5.0.x
+            6.0.x
 
       - name: Cache Tools
         uses: actions/cache@v2

docs/input/docs/index.md

@@ -13,3 +13,5 @@ release, close a milestone, and also to export the complete set of release notes
 for your application/product.
 
 Testing build and publish of docs...
+
+And moar...