[back] sec: update Django from 4.2.15 to 4.2.16 (#2017)

See: https://docs.djangoproject.com/en/4.2/releases/4.2.16/

backend/requirements.txt

@@ -3,7 +3,7 @@
 
 # Full stack Web Framework used for Tournesol's backend
 # https://docs.djangoproject.com
-Django==4.2.15
+Django==4.2.16
 # Used for fields computed on save for Django models
 # See https://github.com/brechin/django-computed-property/
 django-computed-property==0.3.0