Revert “Issue #3048976 by Niklan, markcarver: Upgrade to league/commonmark ^1.0”

This reverts the Drupal 9 portions committed in 54c12a13.
See https://www.drupal.org/project/markdown/issues/3103679#comment-13561137



Signed-off-by: Mark Carver <mark.carver@me.com>