feat(GlKeysetPagination): use tertiary category for buttons

mentioned in commit ebf58857

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1109 (merged)

mentioned in merge request status-page!903

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 gitlab-org/frontend/playground/accessibility-scanner!58 (merged)