*/*dj51*: Deprecate/Set to expire

[kai]

Summary

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

Test Plan

The deprecation messages all refer to the Django 5.2 variants, that should be used. Some py-dj51-* ports do not currently have a corresponding Django 5.2 clone, yet, which will probably be added soon when deskutils/py-paperless-ngx and net-mgmt/peering-manager are switched to Django 5.2.

ETA for landing would be end of September before the 2025Q4 branch is created.

[sunpoet]

LGTM. Thanks!