Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / web / core / modules / migrate_drupal / migrate_drupal.info.yml

name: Migrate Drupal
type: module
description: 'Contains migrations from older Drupal versions.'
package: Migration
version: VERSION
core: 8.x
dependencies:
  - drupal:migrate