feat(GlToast): Add shadow to communicate elevation

mentioned in commit 12b93e20

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1030 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!167068 (closed)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1032 (closed)

mentioned in merge request gitlab!167406 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4084 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!349 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!51 (merged)