[meta] Tasks to remove Toolbar module

Problem/Motivation

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

The removal of extension Toolbar was approved in #3476882: [Policy] Move Toolbar module to contrib.

Remaining tasks

1. ✅Create a child issue, 'Remove Toolbar from core'. #3586211: Remove the Toolbar module
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. ✅Open an issue, 'Ensure that Toolbar does not get special core treatment' in the packages.drupal.org issue queue to ensure that the composer namespace for the contributed project matches the core one after removal. This is to allocate the drupal/Toolbar Composer namespace to contrib extension rather than the core extension.
4. Manually test upgrading from the core extension to the contrib extension. #3586203: Test the upgrade path for the Toolbar module
5. ✅Triage core issues for Toolbar 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.
6. ✅Move all core documentation for Toolbar to the contrib project. Make issues in the contrib project to update or add links to the documentation on the project page.

Release notes snippet