Build with .net9 on build server

Author: Socolin

.github/workflows/dotnet.yml

@@ -53,7 +53,7 @@ jobs:
     - name: Setup .NET
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 8.0.x
+        dotnet-version: 9.0.x
     - name: Restore dependencies
       run: dotnet restore
       working-directory: backend