fix: backend/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-GUNICORN-9510910

Author: snyk-bot

backend/requirements.txt

@@ -15,7 +15,7 @@ email-validator==1.3.1
 fastapi==0.103.1
 # fastapi-jwt-auth==0.6.1 (wrong version cant deploy)
 fastapi-jwt-auth==0.5.0
-gunicorn==20.1.0
+gunicorn==23.0.0
 h11==0.14.0
 httptools==0.5.0
 idna==3.4