Bump gunicorn from 20.1.0 to 21.2.0

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>

requirements/base.txt

@@ -17,7 +17,7 @@ django-recaptcha==3.0.0
 django-bootstrap-breadcrumbs==0.9.2
 
 # Web serving
-gunicorn==20.1.0
+gunicorn==21.2.0
 whitenoise==6.5.0
 
 # Database APIs