Updates docs to reflect changes in CoreML (#9645)

### Summary

* After https://github.com/pytorch/executorch/pull/9483, we should have
CoreML support out of the box for macOS
* Unfortunately, we still need
`backends/apple/coreml/scripts/install_requirements.sh` to use the
[coreml_executorch_runner](https://github.com/pytorch/executorch/tree/main/examples/apple/coreml/executor_runner)
(used for testing)
* I should have caught all the usage

### Test plan
Read