⬆️ Bump gunicorn from 20.1.0 to 21.1.0 in /shifter (#193)

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.1.0 to 21.1.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@20.1.0...21.1.0)

---
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>
Co-authored-by: Toby Such <toby@tobysuch.uk>

shifter/requirements.txt

@@ -1,7 +1,7 @@
 asgiref==3.7.2
 Django==4.2.3
 django-crontab==0.7.1
-gunicorn==20.1.0
+gunicorn==21.1.0
 psycopg==3.1.9
 psycopg-binary==3.1.9
 sqlparse==0.4.4