chore(deps): bump django from 4.2.20 to 4.2.21 in /backend

Bumps [django](https://github.com/django/django) from 4.2.20 to 4.2.21.
- [Commits](django/django@4.2.20...4.2.21)

---
updated-dependencies:
- dependency-name: django
  dependency-version: 4.2.21
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/requirements.txt

@@ -4,7 +4,7 @@ backcall==0.1.0
 certifi==2024.7.4
 chardet==3.0.4
 decorator==4.3.0
-Django==4.2.20
+Django==4.2.21
 django-environ==0.4.5
 django-extensions==2.2.3
 django-redis==4.9.0