*/*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