Rasa: 2.8.9 Release

• #7619: Bump TensorFlow version to 2.6.

This update brings some security benefits (see TensorFlow release notes for details). However, internal experiments suggest that it is also associated with increased train and inference time, as well as increased memory usage.

You can read more about why we decided to update TensorFlow, and what the expected impact is here.

If you experience a significant increase in train time, inference time, and/or memory usage, please let us know in the forum.

Users can no longer set TF_DETERMINISTIC_OPS=1 if they are using GPU(s) because a tf.errors.UnimplementedError will be thrown by TensorFlow (read more here).

:::caution This breaks backward compatibility of previously trained models. It is not possible to load models trained with previous versions of Rasa Open Source. Please re-train your assistant before trying to use this version.

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