Add config dependency on migration entity to cron_migration entities

Problem/Motivation

When a migration entity is removed (for example due to uninstallation of a module), a corresponding cron_migration entity still exists.

Steps to reproduce

Proposed resolution

cron_migration entities should have something like the following in its config:

dependencies:
  config:
    - migrate_plus.migration.name

Remaining tasks

User interface changes

API changes

Data model changes