feat: Add gl-background-filter-blur class

mentioned in commit 701986ef

mentioned in merge request gitlab!88974 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!2896 (merged)

mentioned in merge request status-page!540 (merged)

mentioned in merge request gitlab-docs!2839 (merged)

mentioned in merge request gitlab-docs!2841 (merged)

mentioned in merge request gitlab-docs!2842 (closed)

mentioned in commit af8c81b4