[back] sec: upgrade Django to 4.1.12 (from 4.1.11) (#1805)

Fix:
- CVE-2023-43665

See:
- https://www.djangoproject.com/weblog/2023/oct/04/security-releases/

backend/requirements.txt

@@ -3,7 +3,7 @@
 
 # Full stack Web Framework used for Tournesol's backend
 # https://docs.djangoproject.com
-Django==4.1.11
+Django==4.1.12
 # Used for fields computed on save for Django models
 # See https://github.com/brechin/django-computed-property/
 django-computed-property==0.3.0