Fix issue with latest version of FastAPI

Receiving this error: 

```
  File "/home/runner/work/txtai/txtai/src/python/txtai/api/route.py", line 29, in handler
    response_field=self.secure_cloned_response_field,
AttributeError: 'EncodingAPIRoute' object has no attribute 'secure_cloned_response_field'
```

This attribute was removed in a recent version of FastAPI. Add a fix to handle this change. 