fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-2968205

requirements.txt

@@ -7,7 +7,7 @@ chardet==4.0.0
 charset-normalizer==2.0.4
 cryptography==3.4.7
 defusedxml==0.7.1
-Django==3.2.6
+Django==3.2.15
 django-allauth==0.44.0
 django-appconf==1.0.4
 django-classy-tags==2.0.0