Reconsider margin-bottom for .btn-block

gitlab-foss@3dc0031f (created on 2013-10-21 !!!) introduced margin-bottom: 15px; for .btn-block, but it should be replaced as if Bootstrap 5 deprecated it.

Implementation from Bootstrap 4.6 via https://github.com/twbs/bootstrap/blob/538ea32cb7818d6091858a983b20d181fc5a09a8/scss/_buttons.scss#L129-L133

.btn-block {
  // ...

  // Vertically space out multiple block buttons
  + .btn-block {
    margin-top: $btn-block-spacing-y;
  }
}

Refs.

• The number of .btn-block was increased in !63069 (comment 1104651030)
  • Further updated was done in !97399 (merged)
• gitlab-foss!1514 (merged)
• gitlab-foss@97c72d74
• gitlab-foss@3dc0031f (2013-10-21 !!!)

Bootstrap 5

• Current guide for Block buttons (Bootstrap 5.0): https://getbootstrap.com/docs/5.0/components/buttons/#block-buttons
• PR for deprecation guide (twbs/bootstrap#31995) https://github.com/twbs/bootstrap/pull/31995