Update dotnetcore-build.yml to include .NET 6.0.x/8.0.x/9.0.x

Author: pbhal

.github/workflows/dotnetcore-build.yml

@@ -14,7 +14,10 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v4
       with:
-        dotnet-version: 9.0.x
+        dotnet-version: |
+          6.0.x
+          8.0.x
+          9.0.x
     
     - name: Install dependencies
       run: dotnet restore RulesEngine.sln