fix(GlDropdown): Update split dropdown chevron to match border-color

mentioned in commit 894689f1

mentioned in merge request gitlab!53718 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2254 (merged)

mentioned in merge request status-page!228 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2255 (merged)

mentioned in merge request status-page!230 (merged)

mentioned in merge request gitlab-docs!1529 (merged)

mentioned in commit 8710c891