[Meta] Tasks to deprecate Forum

Update to latest template from the policy.

Because forum is in a quite a few migration I made an issue to check them separately.

There are references to forum in the following

• core/tests/Drupal/KernelTests/Config/Schema/MappingTest.php:
• core/modules/system/tests/src/Functional/Update/Y2038TimestampUpdateTest.php:
• core/modules/node/tests/src/Functional/NodeTypeTest.php:

This could be an issue for comments;

$ grep --exclude-dir={vendor,node_modules,forum,themes} -riw 'forum' core | grep -v phpstan-baseline.neon | grep -v Test | grep -vi migrat | grep -vi "drupal[6|7].php" 
core/lib/Drupal/Core/Extension/InfoParserInterface.php:   * See forum.info.yml for an example of a module .info.yml file.
core/lib/Drupal/Core/Entity/entity.api.php: * - The Forum module defines a content type in node.type.forum.yml and a
core/lib/Drupal/Core/Config/Entity/ConfigDependencyManager.php: * dependencies key. For example, the Forum module provides the forum node type
core/lib/Drupal/Core/Config/Entity/ConfigDependencyManager.php: * and in order for it to be deleted when the forum module is uninstalled it has
core/lib/Drupal/Core/Config/Entity/ConfigDependencyManager.php: * an enforced dependency on the module. The dependency on the Forum module can
core/lib/Drupal/Core/Config/Entity/ConfigDependencyManager.php: * type configuration entity that depends on functionality provided by the Forum
core/lib/Drupal/Core/Logger/RfcLoggerTrait.php: * @see https://groups.google.com/forum/#!topic/php-fig/Rc5YDhNdGz4
core/lib/Drupal/Core/Render/theme.api.php: *     when the forum is rendered, following render array can be used:
core/lib/Drupal/Core/Render/theme.api.php: *       '#theme' => array('forum__' . $tid, 'forum'),
core/lib/Drupal/Core/Render/theme.api.php: *       '#forum' => $forum,
core/lib/Drupal/Core/Render/theme.api.php:  // Kill the next/previous forum topic navigation links.
core/modules/comment/comment.module: * Users can post comments to discuss a forum topic, story, collaborative
core/modules/system/tests/modules/url_alter_test/url_alter_test.install:  // Set the weight of this module to one higher than forum.module.
core/modules/path/path.module:      $element['value']['#description'] = t('Specify the existing path you wish to alias. For example: /node/28, /forum/1, /taxonomy/term/1.');
core/modules/field/src/Entity/FieldStorageConfig.php:   * classic use case for this is node body field storage since Book, Forum, the
core/MAINTAINERS.txt:Forum

And there are plenty of references in themes.
grep --exclude-dir={vendor,node_modules,forum} -riw 'forum' core | grep -v phpstan-baseline.neon | grep -v Test | grep themes | grep -vi migrat | grep -vi "drupal[6|7].php"

Removing #3409384: [11.x] Remove forum CSS from themes and profiles from the Issue Summary. That will be done when Forum is removed.

Thanks to everyone who worked on the child issues to make this happen!

I am adding credit to myself for the overall admin required to keep this on track.