feat(Modal): Use new semantic bg tokens

mentioned in commit f3bee213

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!952 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab!165726 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!954 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4049 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4067 (closed)

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

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