net-mgmt/seafile-server,www/seahub: Deprecate

Updating www/seahub, and especially net-mgmt/seafile-server, is more
difficult than originally anticipated. The current version of www/seahub
also only runs with Django 4.2, which has been end-of-life since April.

Before www/seahub and net-mgmt/seafile-server are marked as BROKEN after
the migration of the affected ports to Django 5.2 [1], and then vanish
from the ports tree at the end of 2026Q2 - which would also break POLA
due the short time period - do the following to buy some time for a
possible update:

• Create required py-dj42-* ports and switch www/seahub to them. These py-dj42-* ports were copied from their original counterparts, are bound to Django 4.2, and have also been adapted for the PEP517 build framework to make them future-proof. Also adjust related entries in MOVED.

• Add CONFLICTS_INSTALL entries to the original counterparts as well where necessary.

• Bump PORTREVISION of www/seahub due changed dependencies.

• Set a suitable EXPIRATION_DATE to have enough time to work on an update. The EXPIRATION_DATE for www/py-django42 will adjusted accordingly afterwards.

PR: 291707 [1]
Reviewed by: ultima (maintainer)
Differential Revision: https://reviews.freebsd.org/D57227