fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-8456315
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-8456316

Author: snyk-bot

requirements.txt

@@ -30,7 +30,7 @@ cycler==0.10.0
 ddt==1.2.1
 decorator==4.0.9
 dj-database-url==0.4.1
-django==1.11.23
+django==4.2.17
 django-admin-sortable2==0.6.10
 django-appconf==1.0.2
 django-attachments==1.0.1