[META] Prepare for Branching of Drupal 9, and supporting D9 on launch.

The current targeted release for drupal 9 is roughly June 2020, and the d9 cycle will likely be longer than usual, so the branch will be opened much earlier than that, which means we have some things to do before that happens.

One major goal of the drupal 8 to drupal 9 upgrade cycle is to eliminate the upgrade pain that exists between major versions, such that users can upgrade from 8 to 9 easily, which means contributed modules can be simultaneously drupal 8 and drupal 9 compatible. Because of this goal we’ll need to evaluate and address everywhere in the infrastructure that makes assumptions about the ‘core compatibility’ being a 1:1 relationship, and either eliminate it or expand it to be 1:many.

Source: https://www.drupal.org/project/issues/rss/infrastructure