Bump django from 3.1.9 to 3.1.12 in /src/backend

Bumps [django](https://github.com/django/django) from 3.1.9 to 3.1.12.
- [Release notes](https://github.com/django/django/releases)
- [Commits](https://github.com/django/django/compare/3.1.9...3.1.12)

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

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

src/backend/requirements.txt

@@ -2,7 +2,7 @@ requests-oauth==0.4.1
 requests-oauthlib==1.3.0
 requests==2.23.0
 
-django==3.1.9
+django==3.1.12
 django-storages==1.9.1
 django-filter==2.2.0
 django-cors-headers==3.2.1