Update cortex maintainers

[danielblando]

Checklist for maintainer updates

• You've provided a link to documentation where the project has approved the maintainer changes.
  https://github.com/cortexproject/cortex/blob/master/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 [email protected] 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.