fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PROTOBUF-3031740

Author: snyk-bot

requirements.txt

@@ -19,4 +19,5 @@ urllib3>=1.25.10
 websockets>=8.1
 yarl>=1.5.1
 mysql-connector-python>=8.0.21
-alt-profanity-check==1.1.1
+alt-profanity-check==1.1.1
+protobuf>=3.20.2 # not directly required, pinned by Snyk to avoid a vulnerability