Move Rita Zhang to emeritus (#9132)

### Why the changes in this PR are needed?

Keep the OPA maintainer roster current as Rita transitions from active
maintainer to emeritus status.

### What are the changes in this PR?

Remove Rita Zhang from the active maintainers table and add her to the
emeritus list.

### Notes to assist PR review:

No tests are needed because this change only updates project maintainer
metadata.

### Further comments:

None.

Signed-off-by: Jaydip Gabani <gabanijaydip@gmail.com>