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

@@ -1,7 +1,7 @@
 flask==2.2.3
 pdf2image==1.16.3
 pyTelegramBotAPI==4.12.0
-gunicorn==20.1.0
+gunicorn==21.2.0
 msal==1.23.0
 flask-login==0.6.2
 flask-session==0.5.0