Fixes to pubmed dataset download function
Hi ! I think we can simply add a new configuration for the 2022 data instead of replacing them.
You can add the new configuration here:
```python
    BUILDER_CONFIGS = [
        datasets.BuilderConfig(name="2021", description="The 2021 annual record", version=datasets.Version("1.0.0")),
        datasets.BuilderConfig(name="2022", description="The 2022 annual record", version=datasets.Version("1.0.0")),
    ]
```

And we can have the URLs for these two versions this way:
```python
_URLs = {
    "2021": f"ftp://ftp.ncbi.nlm.nih.gov/pubmed/baseline/pubmed21n{i:04d}.xml.gz" for i in range(1, 1063)],
    "2022": f"ftp://ftp.ncbi.nlm.nih.gov/pubmed/baseline/pubmed22n{i:04d}.xml.gz" for i in range(1, 1114)]
}
```
and depending on the configuration name (you can get it with `self.config.name`) we can pick the URLs of 2021 or the ones of 2022 and pass them to the `dl_manager` in `_split_generators`

Feel free to ping me if you have questions or if I can help !
Pubmed has updated its settings for 2022 and thus existing download script does not work. 