Bump gunicorn from 20.1.0 to 21.2.0 (#562)

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
...

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

requirements.txt

@@ -1,6 +1,6 @@
 Django==4.2.4
 psycopg2==2.9.7
-gunicorn==20.1.0 #production server
+gunicorn==21.2.0 #production server
 whitenoise==6.5.0 #serving swagger static files
 djangorestframework==3.14.0
 pylint==2.17.5