Replace bootstrap utils with gl- equivalents in billings / subscription_table.vue

Background

The following discussion from !92729 (merged) should be addressed:

• @ekigbo started a discussion: (+1 comment)

  It would be nice to also replace flex-lg-column but it looks like we dont have that defined yet, so i'll skip for now.

Implementation guide

• We should add gl-lg-flex-direction-column to gitlab-ui
• Replace flex-lg-column with gl-lg-flex-direction-column in ee/app/assets/javascripts/billings/subscriptions/components/subscription_table.vue