Fix version to .NET 9

Author: JanneMattila

.github/workflows/ci.yml

@@ -29,7 +29,7 @@ jobs:
       - name: Setup .NET
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: 8.0.x
+          dotnet-version: 9.0.x
 
       - name: Restore dependencies
         run: dotnet restore