Update gunicorn requirement from <21.0,>=19.9.0 to >=19.9.0,<22.0 (#960)

Updates the requirements on [gunicorn](https://github.com/benoitc/gunicorn) to permit the latest version.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](benoitc/gunicorn@19.9.0...21.0.1)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

requirements.txt

@@ -8,7 +8,7 @@ django-cors-headers>=3.11.0,<5.0
 djangorestframework>=3.11.0,<4.0
 django-rest-knox>=4.1.0,<5.0
 django-filter>=2.2.0,<24.0
-gunicorn>=19.9.0,<21.0
+gunicorn>=19.9.0,<22.0
 importlib-metadata==4.13.0
 lxml>=4.6.3,<5.0
 markdown>=3.2.2,<4.0