Update .NET version in workflow to 8.0.x

Author: ellickau

.github/workflows/dotnet-desktop.yml

@@ -67,7 +67,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: '9.0.x'
+          dotnet-version: '8.0.x'
 
       - name: Build
         run: dotnet build ./LibrarySystem.csproj -c Release