fix: samples/misc/opencv_samples/mqtt_cam/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PROTOBUF-15090738

Author: snyk-bot

samples/misc/opencv_samples/mqtt_cam/requirements.txt

@@ -6,4 +6,4 @@ PyYAML>=5.1.2
 streamlit>=0.62.1
 isort>=5.0.3
 opencv-python>=4.1.1.26 # may fail on rpi
-protobuf>=3.20.2 # not directly required, pinned by Snyk to avoid a vulnerability
+protobuf>=6.33.5 # not directly required, pinned by Snyk to avoid a vulnerability