Update gunicorn requirement in /backend

Updates the requirements on [gunicorn](https://github.com/benoitc/gunicorn) to permit the latest version.
- [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
...

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

backend/requirements.txt

@@ -9,7 +9,7 @@ setproctitle>=1.2.2,<1.4.0
 SQLAlchemy>=1.4.27,<2.1.0
 
 # production runner
-gunicorn>=20.1.0,<20.2.0
+gunicorn>=20.1.0,<21.3.0
 
 # profiling and checking
 newrelic>=7.2.2,<8.8.0