Update from keras2onnx to tf2onnx
# What does this PR do?

[keras2onnx](https://github.com/onnx/keras-onnx) has been dead for a while, with the development being made in [tf2onnx](https://github.com/onnx/tensorflow-onnx/tree/6c417b4210285208ba7165068318542d9e3d9656). It is the successor, yet also retrocompatible.

In fact, our tests were failing right at the import level (i.e. `import keras2onnx` fails with recent `keras` versions). These changes fix the tests for TF+ONNX.

Example of test command that gets fixed with this PR:
```RUN_SLOW=1 pytest tests/test_modeling_tf_bert.py::TFBertModelTest::test_onnx_runtime_optimize```