build(deps): bump google-cloud-storage from 1.30.0 to 1.31.0 in /backend

Bumps [google-cloud-storage](https://github.com/googleapis/python-storage) from 1.30.0 to 1.31.0.
- [Release notes](https://github.com/googleapis/python-storage/releases)
- [Changelog](https://github.com/googleapis/python-storage/blob/master/CHANGELOG.md)
- [Commits](googleapis/python-storage@v1.30.0...v1.31.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/requirements.txt

@@ -5,7 +5,7 @@ fakeredis==1.4.3
 Flask==1.1.2
 Flask-Cors==3.0.9
 flask-talisman==0.7.0
-google-cloud-storage==1.30.0
+google-cloud-storage==1.31.0
 gunicorn==20.0.4
 python-dateutil==2.8.1
 pytoml==0.1.21