Update Django to version 6.0

This commit updates Django from 5.2.8 to 6.0, the latest stable release.

The next steps are to:
1. Run 'uv lock' in the backend directory to update dependencies
2. Run the test suite to identify any compatibility issues
3. Address any breaking changes if needed

Key compatibility notes for Django 6.0:
- Python 3.12+ requirement is already met (project uses 3.13.5)
- DEFAULT_AUTO_FIELD is already set to AutoField in settings
- Email code uses standard EmailMultiAlternatives (no custom subclasses)
- No custom ORM expressions with as_sql() methods found

Co-authored-by: Marco Acierno <marcoacierno@users.noreply.github.com>

Author: github-actions[bot]

backend/pyproject.toml

@@ -44,7 +44,7 @@ known-first-party = [
 [project]
 requires-python = ">=3.13.5"
 dependencies = [
-    "django==5.2.8",
+    "django==6.0",
     "django-environ==0.10.0",
     "django-model-utils==5.0.0",
     "django-timezone-field==7.0",