Bump gunicorn from 22.0.0 to 23.0.0 in /example/backend

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 22.0.0 to 23.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@22.0.0...23.0.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-version: 23.0.0
  dependency-type: direct:production
...

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Author: dependabot[bot]