[11.x] [Meta] Tasks to remove Forum from core and move to contrib

Problem/Motivation

This issue is to coordinate and track the steps needed to remove Forum to contrib after it is deprecated. See Remove a core module and move it to a contributed project of the deprecation policy.

The removal of Forum was approved in #1898812: [policy] Deprecate forum module for removal in Drupal 11

Remaining tasks

1. Create a child issue, #3261653: Remove Forum module
   • The change record for this issue should include a link to recommendations page, https://www.drupal.org/node/3223395#s-forum. (For example, the CR for removing HAL)
   • Tag this issue 'Needs release note.'
   • Remove the extension ;-).
   • Update Update path tests as needed. See Writing Automated Update Tests for Drupal 8 (or later) for details.
   • Remove references from core/phpstan-baseline.neon.
   • Remove any spelling words specific to the extension from the dictionary
   • Check for references in @todo.
   • Handle migration tests.
     • In all the functional tests in migrate_drupal_ui make sure that Forum is not installed. Forum should also be removed from the methods getAvailablePaths() and moved to getMissingPaths() in the tests using those methods.
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 Forum 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/Forum Composer namespace to contrib extension rather than the core extension.
4. Manually test upgrading from the core extension to the contrib extension.
5. Triage core issues for Forum in the core queue and move either to the contrib project or the relevant core component.
6. Move all core documentation for Forum to the contrib project.
7. When the issue in https://www.drupal.org/project/project_composer is Fixed update the documentation with the new Composer command.

Release notes snippet