fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-7642790
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-7642791
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-7642813
- https://snyk.io/vuln/SNYK-PYTHON-DJANGO-7642814

requirements.txt

@@ -1,4 +1,4 @@
-Django>=4.0,<6
+Django>=4.2.15
 Sphinx>=1.2.0
 phonenumbers>=7.0.2
 django-phonenumber-field==6.1.0