build(deps): Bump gunicorn from 20.0.3 to 20.0.4 in /backend (#308)

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.0.3 to 20.0.4.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@20.0.3...20.0.4)

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

Author: dependabot-preview[bot]

backend/requirements.txt

@@ -5,7 +5,7 @@ Flask==1.1.1
 Flask-Cors==3.0.8
 flask-talisman==0.7.0
 google-cloud-storage==1.23.0
-gunicorn==20.0.3
+gunicorn==20.0.4
 python-dateutil==2.8.1
 pytoml==0.1.21
 fakeredis==1.0.5