Bump django from 3.1.8 to 3.1.13 in /containers/backend

Bumps [django](https://github.com/django/django) from 3.1.8 to 3.1.13.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.8...3.1.13)

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

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

containers/backend/requirements.txt

@@ -1,4 +1,4 @@
-Django==3.1.8
+Django==3.1.13
 django-cors-middleware==1.5.0
 djangorestframework==3.12.2
 djangorestframework-simplejwt==4.6.0