update Drupal 11 example for maximum compatibility

[tobybellwood]

This PR:

• Adds the https://drupal.org/project/mysql57 module and code snippet to enable installing Drupal 11 on any compatible MySQL/MariaDB database, instead of only MySQL8.0+
• Updates the drush cr command for a safe first execute with no db present
• Runs the post-rollout tasks in bash shell to avoid null comparison idiosyncracies