Bump gunicorn from 20.1.0 to 21.1.0 in /backend

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.1.0 to 21.1.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@20.1.0...21.1.0)

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

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

backend/requirements.txt

@@ -35,10 +35,12 @@ djangorestframework-simplejwt==5.2.2
     # via -r requirements.in
 drf-writable-nested==0.7.0
     # via -r requirements.in
-gunicorn==20.1.0
+gunicorn==21.1.0
     # via -r requirements.in
 markdown==3.4.3
     # via -r requirements.in
+packaging==23.1
+    # via gunicorn
 psycopg2-binary==2.9.6
     # via -r requirements.in
 pyjwt==2.7.0