fix(GlToast): set background opacity to 1

mentioned in commit 2d2e8ea1

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2427 (merged)

mentioned in merge request status-page!311 (merged)

mentioned in merge request gitlab!64619 (closed)

mentioned in merge request gitlab-docs!1959 (closed)

mentioned in merge request gitlab-docs!1968 (closed)