Bump django from 2.2.12 to 2.2.28 in /zusidatenbank

Bumps [django](https://github.com/django/django) from 2.2.12 to 2.2.28.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@2.2.12...2.2.28)

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

Signed-off-by: dependabot[bot] <[email protected]>

zusidatenbank/requirements.txt

@@ -1,4 +1,4 @@
-Django==2.2.12
+Django==2.2.28
 django-analytical==2.2.2
 django-bootstrap3==9.0.0
 django-flatblocks==0.9.4