Add Kahiro Okina to OCM maintainers

[kahirokunn]

I'm a maintainer of Open Cluster Management's addon-framework, approved in:
open-cluster-management-io/community#239

I'm also listed in the Open Cluster Management core maintainer list:
https://github.com/open-cluster-management-io/community/blob/main/MAINTAINERS.md

Checklist for maintainer updates

• You've provided a link to documentation where the project has approved the maintainer changes.
• 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.

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