Revert protobuf<=3.20.1 (#8742)

requirements.txt

@@ -12,7 +12,7 @@ scipy>=1.4.1
 torch>=1.7.0
 torchvision>=0.8.1
 tqdm>=4.64.0
-protobuf<4.21.3  # https://github.com/ultralytics/yolov5/issues/8012
+protobuf<=3.20.1  # https://github.com/ultralytics/yolov5/issues/8012
 
 # Logging -------------------------------------
 tensorboard>=2.4.1