Add requirements.txt protobuf<=3.20.1

[glenn-jocher]

protobuf >=4 is unstable and breaking CI and training. Pinning to 3.20.1 temporarily to resolve. See #8012

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved package dependency management in YOLOv5's CI and requirements.

📊 Key Changes

• Removed the fixed protobuf==3.20.1 version from CI workflow files.
• Added protobuf<=3.20.1 constraint to the requirements.txt file.

🎯 Purpose & Impact

• ⚙️ Purpose: The change standardizes the version constraint for protobuf across different setup configurations, ensuring compatibility and preventing potential issues as noted in a specific YOLOv5 GitHub issue.
• 🚀 Impact: Users can expect more consistent installations and fewer errors related to the protobuf package version when setting up YOLOv5, leading to a smoother experience.

[addiercv]

There is compatibility issue with tensorboard 2.9.1.

ERROR: tensorboard 2.9.1 has requirement protobuf<3.20,>=3.9.2, but you'll have protobuf 3.20.1 which is incompatible.

OS: Ubuntu 20.04 LTS
Python: 3.8.10

I changed protobuf<=3.20.1 to protobuf<3.20.0, tensorboard 2.9.1 installed successfully (all the packages are installed successfully).

[glenn-jocher]

@addiercv <=3.20.1 is fully compatible with <3.20

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