Deprecate farmOS v1 migrations

It might make sense to keep the farm_migrate module, and use it as a home for farmOS-specific Migrate API plugins (we may add more for #3382539: farmOS v2 CSV import module → ...

In which case we don't need to deprecate the whole module, just the specific classes and configurations in it that are for the v1 migrations. We can then remove all the v1 migration code in 3.x and add an update hook to uninstall any of the migrate_plus.migration.* and migrate_plus.migration.* configuration entities.

Drafted a CR here: https://www.drupal.org/node/3382609 →

Opened a follow-up issue here: #3382616: Remove v1 migrations from farmOS 3.x →

Automatically closed - issue fixed for 2 weeks with no activity.