build(deps): Bump google-cloud-storage from 1.17.0 to 1.18.0 in /backend

Bumps [google-cloud-storage](https://github.com/GoogleCloudPlatform/google-cloud-python) from 1.17.0 to 1.18.0.
- [Release notes](https://github.com/GoogleCloudPlatform/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/master/CHANGELOG.md)
- [Commits](googleapis/google-cloud-python@storage-1.17.0...storage-1.18.0)

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Author: dependabot-preview[bot]

backend/requirements.txt

@@ -4,7 +4,7 @@ datadog==0.29.3
 Flask==1.1.1
 Flask-Cors==3.0.8
 flask-talisman==0.7.0
-google-cloud-storage==1.17.0
+google-cloud-storage==1.18.0
 gunicorn==19.9.0
 python-dateutil==2.8.0
 pytoml==0.1.21