fix: requirements.txt to reduce vulnerabilities

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-DJANGORESTFRAMEWORK-7252137
- https://snyk.io/vuln/SNYK-PYTHON-SQLPARSE-6615674

requirements.txt

@@ -17,3 +17,4 @@ unicms-template-italia>=0.4.6
 markdown
 markdownify
 sqlparse>=0.5.0 # not directly required, pinned by Snyk to avoid a vulnerability
+djangorestframework>=3.15.2 # not directly required, pinned by Snyk to avoid a vulnerability