[meta] Tasks to remove Stable 9 theme

Problem/Motivation

Track the steps needed to remove extension Stable 9 from core to contrib. See Remove a core extension and move it to a contributed project of the deprecation policy.

The removal of extension Stable 9 was approved in #3512287: [policy, no patch] Deprecate Stable9 and move to contrib.

Remaining tasks

1. Create a child issue, 'Remove Stable 9 from core'. #3586221: Remove the Stable 9 theme
2. Deprecate the extension in the current minor development branch. If the next major branch is open, the extension can be removed via a separate patch (to avoid dependent patches), otherwise an issue should be opened to remove the extension once the next major branch is open.
3. Manually test upgrading from the core extension to the contrib extension. #3586224: Test the upgrade path for Stable 9 theme
4. Triage core issues for Stable 9 in the core queue and move either to the contrib project or the relevant core component. Restore the status to what it was before it was postponed. Also, is the issue moved to contrib, then move any change record.
5. Move all core documentation for Stable 9 to the contrib project. Make issues in the contrib project to update or add links to the documentation on the project page.
6. When the issue in https://www.drupal.org/project/project_composer is Fixed update the documentation with the new Composer command.

Release notes snippet