Update dotnet.yml

Author: prozolic

.github/workflows/dotnet.yml

@@ -23,6 +23,7 @@ jobs:
         dotnet-version: |
             6.0.x
             8.0.x
+            9.0.x
     - name: Restore dependencies
       run: dotnet restore Utf8StringSplitter.slnx
     - name: Build