ValueError: 'index=True' is only valid when 'orient' is 'split', 'table', 'index', or 'columns'
Thanks for reporting. We are investigating it. This issue is caused by latest `pandas` release 2.1.0 (released yesterday Aug 30).

See: https://github.com/huggingface/datasets/actions/runs/6035484010/job/16375932085?pr=6198
 People using previous releases of `datasets` should pin `pandas` in their local environment:
```
python -m pip install 'pandas<2.1.0'
```
### Describe the bug

Saving a dataset `.to_json()` fails with a `ValueError` since the latest `pandas` [release](https://pandas.pydata.org/docs/dev/whatsnew/v2.1.0.html) (`2.1.0`)

In their latest release we have:

> Improved error handling when using [DataFrame.to_json()](https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.to_json.html#pandas.DataFrame.to_json) with incompatible index and orient arguments ([GH 52143](https://github.com/pandas-dev/pandas/issues/52143))

i.e. an error is now raised for invalid combinations of `index` and `orient`.

This means that unfortunately the custom logic at this line might sometimes lead to contradictions:

https://github.com/huggingface/datasets/blob/029227a116c14720afca71b9b22e78eb2a1c09a6/src/datasets/io/json.py#L96

e.g. for the default case `orient=records` leads to `index=True`, which now raises a `ValueError`

### Steps to reproduce the bug

```python
import datasets


if __name__ == '__main__':

    dataset = datasets.Dataset.from_dict({"A": [1, 2, 3], "B": [4, 5, 6]})
    dataset.to_json("dataset.json")
```

```shell
>>>
ValueError: 'index=True' is only valid when 'orient' is 'split', 'table', 'index', or 'columns'.
```

### Expected behavior

The dataset is successfully saved as `.json`

### Environment info

`python >= 3.9`
`pandas >= 2.1.0`