Refs #25144 -- Revised deprecation timeline: migrations won't become compulsory.

* ``allow_syncdb`` on database routers will no longer automatically become

  ``allow_migrate``.

* The legacy method of syncing apps without migrations will be removed,

  and migrations will become compulsory for all apps. This includes automatic

  loading of ``initial_data`` fixtures and support for initial SQL data.

* Automatic syncing of apps without migrations will be removed. Migrations will

  become compulsory for all apps unless you pass the ``--run-syncdb`` option to

  ``migrate``.

* Support for automatic loading of ``initial_data`` fixtures and initial SQL

  data will be removed.

* All models will need to be defined inside an installed application or

  declare an explicit :attr:`~django.db.models.Options.app_label`.