[Meta] Tasks to remove CKEditor 4 from core and move to contrib

Problem/Motivation

CKEditor 4 will be end-of-life at the end of 2023.

A new CKEditor 5 integration module has been added to core and is currently being stabilized: #3238333: Roadmap to CKEditor 5 stable in Drupal 9

Once it is stable, it must replace the existing CKEditor module for CKEditor 4 in the Standard profile, and CKEditor 4 must be deprecated and moved to contrib. This issue is to track work to remove CKEditor 4 from core after it is deprecated.

Proposed resolution

Start work on blockers for the module's deprecation now, so that we are ready to deprecate it right away once CKEditor 5 is stable.

See Remove a core module and move it to a contributed project

Remaining tasks

1. Remove the CKEditor 4 module from core
   • Needs issue. This issue should handle the migration tests errors, see #2 of #3270437: [meta] Tasks to deprecate the CKEditor 4 module.
2. Open an issue against https://www.drupal.org/project/project_composer.
3. Manually test upgrading from the core module to the contrib module.
4. Move core issues for CKEditor module from the core queue to the contrib module.
5. Move all core documentation for CKEditor module to the contrib project.
6. When the issue in https://www.drupal.org/project/project_composer is fixed, update the documentation with the new composer command.