build(deps): bump gunicorn from 20.1.0 to 21.2.0 in /backend (#2077)

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.1.0 to 21.2.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@20.1.0...21.2.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -5,7 +5,7 @@ Flask==2.2.3
 Flask-Cors==3.0.10
 flask-talisman==1.1.0
 google-cloud-storage==2.7.0
-gunicorn==20.1.0
+gunicorn==21.2.0
 python-dateutil==2.8.2
 pytoml==0.1.21
 redis==5.0.0