Updated C# CI to .NET 8.0.

Author: AngusJohnson

.github/workflows/actions_csharp.yml

@@ -15,7 +15,7 @@ jobs:
     - name: Setup .NET Core
       uses: actions/setup-dotnet@v1
       with: 
-        dotnet-version: '6.0.x'
+        dotnet-version: '8.0.x'
     - name: Install Dependencies
       run: dotnet restore
     - name: Build Solution