Update protobuf requirement from <=3.20.1 to <4.21.3 (ultralytics#8346)

Updates the requirements on [protobuf](https://github.com/protocolbuffers/protobuf) to permit the latest version.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/main/generate_changelog.py)
- [Commits](https://github.com/protocolbuffers/protobuf/commits)

---
updated-dependencies:
- dependency-name: protobuf
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

requirements.txt

@@ -12,7 +12,7 @@ scipy>=1.4.1  # Google Colab version
 torch>=1.7.0
 torchvision>=0.8.1
 tqdm>=4.41.0
-protobuf<=3.20.1  # https://github.com/ultralytics/yolov5/issues/8012
+protobuf<4.21.3  # https://github.com/ultralytics/yolov5/issues/8012
 
 # Logging -------------------------------------
 tensorboard>=2.4.1