Incompatibility with docutils 0.21 and 0.22

## Problem
The `sphinx-csv-filter` addon currently has problems with docutils 0.21. In crate/crate, appendices/compliance.rst:6, an exception is raised:
```
ERROR: One or more indexes of included_cols are not valid. The CSV data does not contain that many columns. [docutils]
```

## Workaround
Downgrade to docutils 0.20 until the problem has been resolved.

## References
- https://github.com/crate/crate/blob/master/docs/appendices/compliance.rst?plain=1#L23-L29
- https://github.com/crate/crate/blob/master/server/src/main/resources/sql_features.tsv
- https://github.com/crate/crate/pull/18862
- https://github.com/crate/crate/pull/18999

Start by reproducing the failure described at crate/crate's docs/appendices/compliance.rst:6 and inspect the referenced server/src/main/resources/sql_features.tsv and linked pull requests for the changed CSV shape. Done means the sphinx-csv-filter addon works with docutils 0.21 and 0.22 without the included_cols exception, while retaining compatibility with 0.20.

Written by the indexing model from the issue text.