Drupal 8/9/10 compatibility

Problem/Motivation

RedHen CRM has a release candidate (compatible with Drupal ^8.7.7 || ^9) and the Zen theme system has a release candidate (compatible with Drupal ^8.8 || ^9), but the Poultry theme is only available for Drupal 7.

Steps to reproduce

Create a new Composer-managed Drupal 9 project:
$ composer create-project drupal/recommended-project poultry-fail

Change directory into the project root:
$ cd poultry-fail

Composer require RedHen CRM and Entity API dependency:

$ composer require 'drupal/redhen:^1.0@RC'
$ composer require 'drupal/entity'

Attempt to Composer require Poultry theme:
$ composer require 'drupal/poultry:^1.0'

As expected, this will fail:

Updating dependencies
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Root composer.json requires drupal/poultry, it could not be found in any version, there may be a typo in the package name.

Potential causes:
 - A typo in the package name
 - The package is not available in a stable-enough version according to your minimum-stability setting
   see <https://getcomposer.org/doc/04-schema.md#minimum-stability> for more details.
 - It's a private package and you forgot to add a custom repository to find it

Read <https://getcomposer.org/doc/articles/troubleshooting.md> for further common problems.

Installation failed, reverting ./composer.json and ./composer.lock to their original content.
composer [require drupal/poultry:^1.0] failed, composer command failed: exit status 2.

Proposed resolution

Install the latest alpha release of the Zen theme system (currently 8.x-7.0-alpha15). Create a Zen subtheme compatible with RedHen, using the PHP template overrides from the Drupal 7 release as a basis for the new Twig template overrides.

Remaining tasks

Nothing has been done yet; all tasks remain. This issue queue is intended as a call for collaborators.