drush_log() is deprecated and make it drupal 9 compatible

Drupal 9 requires Drush 10, so the drush8 command needs to be ported instead of using the Drupal logger, that one has nothing to do with Drush.

You can find a blogpost about porting to Drush 9 here (same for 10): https://weitzman.github.io/blog/port-to-drush9

Drush8 command can be left in the module, but doesn't need any changes as the code is not deprecated for Drupal 8 and Drush 8.