MDL-52846 tasks: complete docs and move functions to deprecatedlib

- Add MDL codes to @deprecated.
- Add @todo tag for future 4.3 phase 2 deprecation.
- Deprecate global scope functions:
  - cron_execute_plugin_type().
  - cron_bc_hack_plugin_functions().
  (moving them to deprecatelib).
- Document it in main upgrade.txt

lib/classes/task/legacy_plugin_cron_task.php

@@ -27,7 +27,8 @@ namespace core\task;
  * Simple task to run cron for all plugins.
  * Note - this is only for plugins using the legacy cron method,
  * plugins can also now just add their own scheduled tasks which is the preferred method.
- * @deprecated since Moodle 3.5 - Please use new task API.
+ * @deprecated since Moodle 3.9 MDL-52846. Please use new task API.
+ * @todo MDL-61165 This will be deleted in Moodle 4.3
  */
 class legacy_plugin_cron_task extends scheduled_task {