Bump django from 3.0.7 to 3.1.6 in /containers/backend (#605)

Bumps [django](https://github.com/django/django) from 3.0.7 to 3.1.6.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.0.7...3.1.6)

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

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

containers/backend/requirements.txt

@@ -1,4 +1,4 @@
-Django==3.0.7
+Django==3.1.6
 django-cors-middleware==1.5.0
 djangorestframework==3.11.0
 djangorestframework-simplejwt==4.6.0