Now that #17921 was merged, we need to update the maintainers doc for the new release process:

https://scikit-learn.org/dev/developers/maintainer.html#releasing

/cc @alfaro96

In particular we should mention that it's important to make sure that the pyproject.toml file is up to date and using the oldest version of numpy for each python version (and don't forget to add a new line if we support a new version of Python) as is being done in #18761.