Case C-660/20, Lufthansa CityLine: Judgment of the Court (First Chamber) of 19 October 2023 (request for a preliminary ruling from the Bundesarbeitsgericht — Germany) — MK v Lufthansa CityLine GmbH (Reference for a preliminary ruling — Social policy — Part-time work — Directive 97/81/EC — Framework Agreement on part-time work concluded by UNICE, CEEP and the ETUC — Clause 4.1 — Principle of non-discrimination of part-time workers — Principle pro rata temporis — Pilots — Remuneration for additional flying duty hours — Identical trigger thresholds for full-time and part-time pilots — Difference in treatment)