Fix polars version

[mdahlhausen]

Pull request overview

Fix the polars version to 0.20.7. Neither the setup requirement of 0.19.16 or the latest 0.20.30 work with comstock postprocessing currently. Update to the latest polars version once someone has time to go through the changes.

Pull Request Author

This pull request makes changes to (select all the apply):

• [ ] Documentation
• [ ] Infrastructure (includes singularity image, buildstock batch, dependencies, continuous integration tests)
• [ ] Sampling
• [ ] Workflow Measures
• [ ] Upgrade Measures
• [ ] Reporting Measures
• [x] Postprocessing

Author pull request checklist:

• [x] Tagged the pull request with the appropriate label (documentation, infrastructure, sampling, workflow measure, upgrade measure, reporting measure, postprocessing) to help categorize changes in the release notes.

Review Checklist

This will not be exhaustively relevant to every PR.

• [ ] Perform a code review on GitHub
• [ ] If fixing a defect, verify by running develop branch and reproducing defect, then running PR and reproducing fix