ci: update leaderboard workflow for Node 24

[Thecesar85]

Summary

• update actions/checkout from v4 to v5 in the update-pr-leaderboard workflow
• proactively align the workflow with GitHub Actions' Node 24 transition

Why

GitHub is close to switching JavaScript actions to Node 24 by default, and the current workflow emits a deprecation warning for actions/checkout@v4. Since the transition date is close, I decided to improve the workflow now and remove that warning path before it becomes operational noise or a future breakage point.

Validation

• workflow change is limited to the checkout action version
• no behavior change intended beyond the runtime compatibility update

[Thecesar85]

Given how close the Node 24 switchover date is, I decided to clean this warning path up proactively. This keeps the leaderboard workflow quieter now and reduces the chance of a near-term maintenance surprise once GitHub flips the default runtime.