Bump flask-cors from 3.0.3 to 3.0.9 in /backend

Bumps [flask-cors](https://github.com/corydolphin/flask-cors) from 3.0.3 to 3.0.9.
- [Release notes](https://github.com/corydolphin/flask-cors/releases)
- [Changelog](https://github.com/corydolphin/flask-cors/blob/master/CHANGELOG.md)
- [Commits](corydolphin/flask-cors@3.0.3...3.0.9)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

backend/requirements.txt

@@ -1,6 +1,6 @@
 flask>=0.12.3
 gevent==20.6.2
-Flask-Cors==3.0.3
+Flask-Cors==3.0.9
 Flask-Security==3.0.0
 Flask-SQLAlchemy==2.4.3
 git+https://github.com/ansrivas/pylogging.git