Remove hacking license tags when mirroring datasets on the Hub
_The documentation is not available anymore as the PR was closed or merged._ The Hub doesn't allow these characters in the YAML tags, and git push fails if you want to push a dataset card containing these characters. Ok, let me rename the bad config names :) I think I can also keep backward compatibility with a warning Almost done with it btw, will submit a PR that shows all the configuration name changes (from a bit more than 20 datasets) Please, let me know when the renaming of configs is done. If not enough bandwidth, I can take care of it... Will focus on this this afternoon ;) I realized when renaming all the configurations with dots in https://github.com/huggingface/datasets/pull/4365 that it's not ideal for certain cases. For example:
- many configurations have a version like "1.0.0" in their names
- to avoid breaking changes we need to replace dots with underscores in the user input and show a warning, which hurts the experience
- our second most downloaded dataset at the moment is affected: `newsgroup`
- if we disallow dots, then we'll never be able to make the [allenai/c4](https://huggingface.co/datasets/allenai/c4) work with its different configurations since they contain dots, and we can't rename them because they are the official download links

I was thinking of other alternatives:
1. just stop separating tags per config name completely, and have a single flat YAML for all configurations. Dataset search doesn't use this info anyway
2. use another YAML structure to avoid having config names as keys, such as
```yaml
languages:
- config: 20220301_en
  values:
  - en
```

I'm down for 1, to keep things simple @lhoestq I agree:
- better not changing config names (so that we do not introduce any braking change)
- therefore, we should not use them as keys

In relation with the proposed solutions, I have no strong opinion:
- option 1 is simpler and aligns better with current usage on the Hub (configs are ignored)
- however:
  - we will lose all the information per config we already have (for those datasets containing config keys; contributors made an effort to put that information per config)
  - and this information might be useful on the Hub in the future, in case we would like to enrich the search feature with more granularity; this is only applicable if this feature could eventually make sense

So, no strong opinion... Closing in favor of https://github.com/huggingface/datasets/pull/4367
Currently, when mirroring datasets on the Hub, the license tags are hacked: removed of characters "." and "$". On the contrary, this hacking is not applied to community datasets on the Hub. This generates multiple variants of the same tag on the Hub. 

I guess this hacking is no longer necessary:
- it is not applied to community datasets
- all canonical datasets are validated by maintainers before being merged: CI + maintainers make sure license tags are the right ones

Fix #4298.