fix: backend/requirements/base.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-6370660

backend/requirements/base.txt

@@ -8,7 +8,7 @@ hiredis==1.0.1  # https://github.com/redis/hiredis-py
 
 # Django
 # ------------------------------------------------------------------------------
-django==3.0.7  # pyup: < 3.1  # https://www.djangoproject.com/
+django==3.2.25  # pyup: < 3.1  # https://www.djangoproject.com/
 django-environ==0.4.5  # https://github.com/joke2k/django-environ
 django-model-utils==4.0.0  # https://github.com/jazzband/django-model-utils
 django-allauth==0.42.0  # https://github.com/pennersr/django-allauth