Add Cedar maintainers to project-maintainers.csv

[riaankleinhans]

Checklist for maintainer updates

Note

Delete this template if you're not changing the CSV file

• You've provided a link to documentation where the project has approved the maintainer changes. https://github.com/cedar-policy/.github/blob/main/profile/MAINTAINERS.md
• The maintainer(s) also created or updated their LFX Individual Dashboard profile.
• You've sent an email with the list of email address(es) to cncf-maintainer-changes@cncf.io for invitations to Service Desk and mailing lists. You can just mark this complete if you are only removing people.
• Optional: You've also sent a PR with affiliation updates to cncf/gitdm.

[riaankleinhans]

@john-h-kastner-aws can you please review the PR?