Bump django from 3.0.7 to 3.1.14 in /backend

Bumps [django](https://github.com/django/django) from 3.0.7 to 3.1.14.
- [Release notes](https://github.com/django/django/releases)
- [Commits](django/django@3.0.7...3.1.14)

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

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

backend/requirements.txt

@@ -3,7 +3,7 @@ astroid==2.3.3
 certifi==2020.6.20
 chardet==3.0.4
 dj-database-url==0.5.0
-Django==3.0.7
+Django==3.1.14
 django-common-helpers==0.9.2
 django-cors-headers==3.2.0
 django-cors-middleware==1.5.0