Deprecate migrate process plugins

Updated from discussion on the Migrate video call today. benjifisher, heddn, mikelutz and quietone were present. 📌 [meeting] Migrate Meeting 2025-04-24 2100Z Active

Two of the blocking issues have been Fixed, and the third is Closed (won't fix).

I suspect: https://git.drupalcode.org/project/drupal/-/blob/11.x/core/modules/filte... will be on the list for removal.

Yes, the filter_id process plugin is needed only when upgrading from Drupal 6 or 7. It should be deprecated as part of this issue.

I am updating the title to clarify that we are not planning to deprecate all process plugins. My title is a little awkward, so feel free to improve on it.