Bump main version to 0.21.0

[jtcohen6]

Port 143c39a to main

This will at least unblock #229, while we continue to work through pre-v1 readiness (#227)

[cla-bot]

Thank you for your pull request and welcome to our community. We could not parse the GitHub identity of the following contributors: Leah Antkiewicz.
This is most likely caused by a git client misconfiguration; please make sure to:

1. check if your git client is configured with an email to sign commits git config --list | grep email
2. If not, set it up using git config --global user.email email@example.com
3. Make sure that the git commit email is configured in your GitHub account settings, see https://github.com/settings/emails