feat(GlProgressBar): Replace Bootstrap Background Class with Custom CSS

mentioned in commit 3eeb5ebe

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1177 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4129 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4146 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4147 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1249 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)