remove the non-exist field updated_at

[yangw-dev]

Description

Remove non-exist field in the hearbeat update logic

even this has error, it does not affect the functionality of submission now, but may make us lose track of hearbeat of the job,

It's being used here, but exception is silence since it only used for hearbeat tracking
https://github.com/gpu-mode/discord-cluster-manager/blob/a992037eeb8763ce7b1f4d69ea06e41cf93b39c3/src/libkernelbot/background_submission_manager.py#L221

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.

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/libkernelbot
leaderboard_db.py					378-387
Project Total

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