unpin slack_sdk version (#18901)

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

.github/workflows/self-push.yml

@@ -533,5 +533,5 @@ jobs:
         # We pass `needs.setup.outputs.matrix` as the argument. A processing in `notification_service.py` to change
         # `models/bert` to `models_bert` is required, as the artifact names use `_` instead of `/`.
         run: |
-          pip install slack_sdk==3.18.1
+          pip install slack_sdk
           python utils/notification_service.py "${{ needs.setup.outputs.matrix }}"