Bump django from 2.2.27 to 2.2.28 in /requirements

Bumps [django](https://github.com/django/django) from 2.2.27 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/2.2.27...2.2.28)

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

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

requirements/requirements.txt

@@ -1,4 +1,4 @@
-django==2.2.27
+django==2.2.28
 django-haystack==3.1.1
 django-filter==2.4.0
 djangorestframework==3.12.4