Upgrade dotnet from 6 to 8 in GitHub workflow (#250)

Author: sarkikos

.github/workflows/dotnet.yml

@@ -16,7 +16,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: 6.0.x
+        dotnet-version: 8.0.x
     - name: Restore dependencies
       run: dotnet restore
       working-directory: aspnetcore/