rename problem table to leaderboard

[b9r5]

Description

The table for storing data about leaderboard challenges was named problem. This didn't seem like a good name, and @alexzhang13 expressed a preference for the name leaderboard.

Checklist

Before submitting this PR, ensure the following steps have been completed:

• Run the slash command /verifyruns on your own server.
  • Run the cluster bot on your server:

    python discord-bot.py

  • Start training runs with the slash command /verifyruns.
  • Verify that the bot eventually responds with:

    ✅ All runs completed successfully!
    

    (It may take a few minutes for all runs to finish. In particular, the GitHub
    runs may take a little longer. The Modal run is typically quick.)
    For more information on running a cluster bot on your own server, see
    README.md.

[alexzhang13]

All looks good. Thanks for the change!