Update to use short array syntax

Tagging this issue as novice and badcamp2019 to use in the first time contributors WS. Please hold for BADCamp, October 2-5

The file bootstrap_sass.theme contains arrays that are still using the old array syntax $var = array('foo', 'bar', 'baz');

These should be updated to the new short array syntax that is part of the Drupal Coding Standards.