feat(GlButton): Update selected state background

mentioned in commit c52c94d9

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2746 (closed)

mentioned in merge request status-page!472 (merged)

mentioned in merge request gitlab!82590 (closed)

mentioned in commit 2ef3d8ff

mentioned in merge request gitlab-docs!2490 (merged)

mentioned in merge request gitlab!83216 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2755 (merged)