*/*dj51*: Deprecate/Set to expire

• Django 5.1 will reach its End-of-Life in December 2025. Deprecate its consumers and set the expiration date accordingly.

• Note: py-dj51-django-netfields does not currently have a corresponding Django 5.2 clone, yet, which will probably be added soon when net-mgmt/peering-manager is switched to Django 5.2.

Reviewed by: sunpoet
With hat: python
Differential Revision: https://reviews.freebsd.org/D52511