Incorrect core dependency in composer.json

Had the same issue and had to pin to a specific version in composer.json.

This fixes the issue and https://www.drupal.org/docs/develop/using-composer/add-a-composerjson-file → recommends not having the core requirement.

I assume this won't stop composer bringing in 2.1.0 / 2.1.1 for D9 unless they're removed somehow and re-released?