[DX] Add a legacy upgrade_mappings database table

Describe your issue or idea

This is a follow-up to https://github.com/backdrop/backdrop-issues/issues/2842 where we realized that a lot of people were upgrading from Drupal to Backdrop in a way that will result in broken sites.

The root of the problem, it seems, is that people want to update core before contrib, and contrib modules often require their updates to be run between two core updates.

The most common use-case for a contrib module needing run an update before core, is when a core database table is dropped. There are 3 places where we already know this to be an issue faced by Backdrop contrib: * Role IDs * Vocabulary VIDs * Views

Would it ease the process of updating if we were to store a mapping of the old Drupal 7 keys to new Backdrop keys in a separate table, that should only be used during the update process?

Having an extra database table would keep our config files clean of dangerous or incomplete data, but might still provide a tool for contrib modules to complete their updates after core, even though the old ID columns (and their database tables) have already been deleted.