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

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----> 1 december_day.replace(month=today.month+1)

ValueError: month must be in 1..12