Update workflows to use .NET 8.0 also. (#1681)

.github/workflows/test-dot-net-nuget.yaml

@@ -75,10 +75,10 @@ jobs:
         run: |
           df -h
 
-      - name: Setup .NET 6.0
+      - name: Setup .NET 8.0
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: 6.0.x
+          dotnet-version: 8.0.x
 
       - name: Check dotnet
         run: dotnet --info