Revert protobuf<=3.20.1

[glenn-jocher]

Resolve #8012 (again)

🛠️ PR Summary

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🌟 Summary

Adjustment of protobuf version requirements for better compatibility.

📊 Key Changes

• Modified the required protobuf version in requirements.txt from <4.21.3 to <=3.20.1.

🎯 Purpose & Impact

• 🛠 This change aims to address compatibility issues highlighted in issue Cant start training: TypeError: Descriptors cannot not be created directly. #8012.
• 👩‍💻👨‍💻 Users can expect more stable installations and fewer conflicts with this specific dependency when setting up YOLOv5.
• 🔄 Ensuring a specific version of protobuf may prevent potential bugs that could arise from newer, untested versions.