message: Update release notes with a more explicit "Breaking changes" section for keras code.
diff: * `tf.train.experimental.enable_mixed_precision_graph_rewrite` is removed, as the API only works in graph mode and is not customizable. The function is still accessible under `tf.compat.v1.mixed_precision.enable_mixed_precision_graph_rewrite`, but it is recommended to use the [Keras mixed precision API](https://www.tensorflow.org/guide/mixed_precision) instead.
  * `tf.lite`:
  +
  * Remove `experimental.nn.dynamic_rnn`, `experimental.nn.TfLiteRNNCell` and `experimental.nn.TfLiteLSTMCell` since they're no longersupported. It's recommended to just use [keras lstm](https://www.tensorflow.org/api_docs/python/tf/keras/layers/LSTM) instead.
  +* Keras been split into a separate PIP package (`keras`), and its code has been moved to the GitHub repository[keras-team/keras](http://github.com/keras-team/keras). The API endpoints for `tf.keras` stay unchanged, but are now backed by the `keras` PIP package. The existing code in tensorflow/python/keras is a staled copy and will be removed in future release (2.7). Please remove any imports to `tensorflow.python.keras` and replace them with public tf.keras API instead.
  +
  ## Known Caveats
  * TF Core:
  