Bump gunicorn from 21.2.0 to 22.0.0 in /backend

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 21.2.0 to 22.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@21.2.0...22.0.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/requirements.txt

@@ -64,7 +64,7 @@ frozenlist==1.4.1
     #   aiosignal
 greenlet==3.0.3
     # via sqlalchemy
-gunicorn==21.2.0
+gunicorn==22.0.0
     # via -r requirements.in
 howlongtobeatpy==1.0.5
     # via -r requirements.in