Bump protobuf in /NLP/Conversational-Recommendation-BASELINE

Bumps [protobuf](https://github.com/protocolbuffers/protobuf) from 3.10.0 to 3.15.0.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.10.0...v3.15.0)

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

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

NLP/Conversational-Recommendation-BASELINE/requirements.txt

@@ -28,7 +28,7 @@ opencv-python==4.1.1.26
 paddlepaddle-gpu==1.6.1.post97
 Pillow==6.2.0
 prettytable==0.7.2
-protobuf==3.10.0
+protobuf==3.15.0
 pyglet==1.3.2
 pyparsing==2.4.2
 python-dateutil==2.8.0