Upgrade Django to 4.2.16

[willbarton]

This updates Django to 4.2.16, which addresses two CVEs, one moderate and one low: https://docs.djangoproject.com/en/5.1/releases/4.2.16/

Checklist

• [x] PR has an informative and human-readable title
• [x] Changes are limited to a single goal (no scope creep)