Deprecate preprocess variable cruft in core

📌 Deprecate $variables['teaser'] Fixed got committed.

Opened 📌 Deprecate $variables['page'] for node.html.twig Active .

I think we also have some node template variables that are documented but no longer exist:

 * - metadata: Metadata for this node.
 * - readmore: Flag for more state. Will be true if the teaser content of the
 *   node cannot hold the main body content.

Additionally these are set by template_preprocess and are not node specific, so can be removed from the docs.

 * - logged_in: Flag for authenticated user status. Will be true when the
 *   current user is a logged-in member.
 * - is_admin: Flag for admin user status. Will be true when the current user
 *   is an administrator.