adds jcooklin as crossplane maintainer

[jcooklin]

Checklist for maintainer updates

• You've provided a link to documentation where the project has approved the maintainer changes.
  • https://github.com/crossplane/crossplane/blob/main/GOVERNANCE.md#steering-committee-1
• 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.