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

[kai]

Summary

Updating www/seahub, and especially net-mgmt/seafile-filer, 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-filer are marked as BROKEN after
the migration of the affected ports to Django 5.2, and then disappear
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 work on an update.
  The EXPIRATION_DATE for www/py-django42 will adjusted accordingly
  afterwards.

PR:             291707

Test Plan

• poudriere -> OK (14.3-RELEASE amd64)