Update min versions in README and add Flax
# What does this PR do?

This PR adapts the minimum versions of each backend (PyTorch was still at 1.0 and TensorFlow at 2.0), removes mention of TensorFlow 2.0 to just say TensorFlow (I think it's safe now!) and adds Jax as an official backend since we have worked the API a bit more.

Fixes #11422