fix(GlLoadingSpinner): restore spin CSS animation

mentioned in commit 6d95633d

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!55 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!352 (merged)

mentioned in merge request status-page!903

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1269 (merged)

mentioned in merge request gitlab!172936 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1279 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4184 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4185 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!58 (merged)