Update autodownload fallbacks to v6.0 assets

[glenn-jocher]

Minor bug fix, this should have been a part of release v6.0 PR.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Update fallback download assets and release tag in downloads.py.

📊 Key Changes

• Added 'yolov5n.pt' and 'yolov5n6.pt' to the list of fallback assets.
• Updated the fallback release tag from 'v5.0' to 'v6.0'.

🎯 Purpose & Impact

• Ensures the availability of the newest model weights (yolov5n and yolov5n6) in case the primary method of fetching release assets fails. 🆕
• Reflects the latest release version in the fallback plan, which aids in maintaining the relevance and up-to-dateness of the download script. 🔄
• Users reliant on the fallback will have access to the latest models, enhancing their experience with more recent and potentially improved pre-trained weights. 💪