fix: api/requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-FLASKCORS-6670412

api/requirements.txt

@@ -9,7 +9,7 @@ entrypoints==0.3
 flake8==3.7.9
 flasgger==0.9.4
 Flask==1.1.2
-Flask-Cors==3.0.8
+Flask-Cors==4.0.1
 flask-mongoengine==0.9.5
 Flask-WTF==0.14.3
 importlib-metadata==1.6.0