fix: requirements/test.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

requirements/test.txt

@@ -21,3 +21,4 @@ didkit==0.2.1
 datar==0.8.6
 pdtypes==0.0.4
 importlib-metadata==4.13.0
+django>=4.2.17 # not directly required, pinned by Snyk to avoid a vulnerability