Update dotnet version to 9.x.x in build workflow

Signed-off-by: Christoph Hofmann <oss@hoffe.org>

Author: hoffe86

.github/workflows/build.yml

@@ -49,7 +49,7 @@ jobs:
     - name: Setup dotnet
       uses: actions/setup-dotnet@v3
       with:
-        dotnet-version: '8.x.x'
+        dotnet-version: '9.x.x'
 
     - name: Set up JDK 17
       uses: actions/setup-java@v3