Send localized `start_time` and `end_time` to Zetkin

Zetkin is handling the start_time and end_time the wrong way, this is a workaround with this issue. Zetkin requires the timestamps for start_time and end_time to be naiv and localized to Germany.

This means:

'2025-01-31T13:58:07.620Z' (UTC, as indicated by Z) becomes 2025-01-31T14:58.07.620 (no timezone).

Remark: In contrast: published and cancelled require an ISO-timestamp.

ref: * die-linke-app-frontend#53 * die-linke-app-frontend#93 * https://gitlab.die-linke.de/die-linke-app/die-linke-app-frontend/-/issues/93#note_2922