Fix first_day_next_month issue
CHANGELOG:
### Fixed
- [#1137](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1137) Fix first_day_next_month problem with december month
Related card: https://trello.com/c/XXXXXXXX/xjksdjflsdfksldfsdfs
----> 1 december_day.replace(month=today.month+1)
ValueError: month must be in 1..12
Edited by Carla Berenguer