Allow marking theme templates as obsolete

Problem/Motivation

📌 Provide a mechanism to mark entire twig templates as deprecated Fixed allows marking templates as deprecated. However, we need a second step to mark theme templates as obsolete so that we can trigger deprecation for any remaining templates and preprocess functions.

Steps to reproduce

Proposed resolution

/**
  * Implements hook_theme().
  */
function mymodule_theme() {
  return [
    'mymodule_custom_template' => [
      'obsolete' => TRUE, // Or possibly a custom message?
    ],
  ];
}

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet