Deprecate dupe cron_run_single_task() function

Manual testing steps: Put into your local directory the attached test file  test_cron_run_single_task.php  (This script execute \mod_forum\task\cron_task using the deprecated function cron_run_single_task). Log in as an admin Go to Site administration / Server / System paths Set " Path to PHP CLI " to the path to your PHP executable. On Linux, you can use the value returned by the command "which php" on your terminal. On Windows, you can use the value returned by the command "where php" on your terminal. Go to this URL in your browser: http://MOODLESITE/local/test_cron_run_single_task.php Expected result: Check you can see this error message: cron_run_single_task() is deprecated. Please use \tool_task\run_from_cli::execute() instead.   line 3630 of /lib/deprecatedlib.php: call to debugging() line 11 of /local/test_cron_run_single_task.php: call to cron_run_single_task()   Execute scheduled task: Forum mailings and maintenance jobs (mod_forum\task\cron_task) Starting digest processing...