chore(github): update reactionSchema (#69203)
## Why?
The GitHub documentation schema for issues → opened is incorrect for
`reactions`. Not entirely sure if this will fix the errored action
(error is truncated?), but this was the only issue I found.
-
https://docs.github.com/en/webhooks/webhook-events-and-payloads?actionType=opened#issues
-
https://github.com/vercel/next.js/actions/runs/10510363688/job/29118781461
