fix(contract_tariff_change): always set date_end to newest line
CHANGELOG:
### Fixed
- [#1106](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1106) Always set date_end to lastest line when executing the change tariff wizard
Related card: https://trello.com/c/L0uQTsjo/1743-revisar-perqu%C3%A8-es-poden-generar-casos-amb-dues-tarifes-actives-al-mateix-temps