Overwrite legacy default config name in `dataset_infos.json` in packaged datasets
Currently if we push data as default config with `.push_to_hub` to a repo that has a legacy `dataset_infos.json` file containing a legacy default config name like `{username}--{dataset_name}`, new key `"default"` is added to `dataset_infos.json` along with the legacy one. I think the legacy one should be dropped in this case.

Also, in `load.py` I suggest to check if a legacy config name is indeed a legacy config name because after this fix it might not be the case (this check was first introduced in https://github.com/huggingface/datasets/pull/6218)
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6231). All of your documentation changes will be reflected on that endpoint. realized that this pr is still not merged, @lhoestq maybe you can take a look at it?  I think https://github.com/huggingface/datasets/pull/6218 fixed the issue (a bit differently though) ah actually nope, let me check @lhoestq yeah the pr you're referencing doesn't fix the problem when two semantically analogous configs occur in datasets_info.json, i suggest to rewrite the legacy one if it exists during .push_to_hub Only the old versions of `datasets` use the JSON file over the README and they can only load one config so the name doesn't really matter.

That's why I chose to load the info from the JSON no matter the name (no check to see if it's "username--dataset_name") in my previous PR.

I think you can remove the old info without even checking the name. In this case maybe no need to update load.py  (also minor: not checking the name makes it more robust to dataset renaming) @lhoestq okay makes sense... so you think it's not a problem that in some cases we might end up with `dataset_infos.json` having two keys in it? > @lhoestq okay makes sense... so you think it's not a problem that in some cases we might end up with dataset_infos.json having two keys in it?

Ideally they should have only one config no ? Since old versions of `datasets` simply load the first config in the JSON.
We can overwrite it with the new default one (and no matter the name of the outdated config in the JSON)

