Change start_date from new contract when changing holder
CHANGELOG:
### Changed
- [#997](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/997) When changing a contract's holder, the new contract should start one day after the end date from the old one.
Related card: https://trello.com/c/mPYJviEs/1511-data-inici-contractes-creats-a-partir-dun-canvi-de-titular