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.txt

@@ -6,7 +6,7 @@ Flask-SSLify==0.1.5
 Flask-Testing==0.8.1
 Flask-SeaSurf==1.1.1
 flask-talisman==1.0.0
-gunicorn==20.1.0
+gunicorn==21.2.0
 pycodestyle==2.10.0
 pydocstyle==6.3.0
 requests==2.31.0