[host_callback] Flip the JAX_HOST_CALLBACK_LEGACY flag to False
`jax.experimental.host_callback` has been deprecated since March 2024
(JAX version 0.4.26). Now we set the default value of the `--jax_host_callback_legacy` configuration value to `True`, which means that if your code uses `jax.experimental.host_callback` APIs, those API calls will be implemented in terms of the new `jax.experimental.io_callback` API.
If this breaks your code, for a very limited time, you can set the `--jax_host_callback_legacy` to `True`. Soon we will remove that configuration option, so you should instead transition to using the new JAX callback APIs.
See https://github.com/google/jax/issues/20385 for a discussion.
PiperOrigin-RevId: 681004255
