fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-GUNICORN-6615672

requirements.txt

@@ -6,7 +6,7 @@ Flask-SSLify==0.1.5
 Flask-Testing==0.8.1
 Flask-SeaSurf==1.1.1
 flask-talisman==1.1.0
-gunicorn==21.2.0
+gunicorn==22.0.0
 pycodestyle==2.11.1
 pydocstyle==6.3.0
 requests==2.31.0