Provide a variable for testing previous version of Drupal core

Migrated issue

Reported by: dave reid

Related to !61 (merged)

Problem/Motivation

For modules that want to maintain compatibility with two major versions of Drupal core at the same time (9 and 10, 10 and 11), it would be helpful to have a variable for "previous version of Drupal core" so that we are not responsible for hard-coding that core version ourselves. We can see this has to be done manually in https://git.drupalcode.org/project/keycdn/-/blob/8.x-1.x/.gitlab-ci.yml#L26

Proposed resolution

* Add new variables for previous Drupal core version to help replicate the options allowed by DrupalCI:

Options offered by DrupalCI for previous versions of core

Remaining tasks

* Submit a MR with the change.

Related issue: Issue #3396005

Related issue: Issue #3396030

Edited by drupalbot