Adapt contract_group to somconnexio

CHANGELOG:

### Add
- [#1151](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/1151) Adapt contract_group to somconnexio

Related card: https://trello.com/c/x7AxcWn4/1805-projecte-facturaci%C3%B3-definir-nom-i-code-dels-contract-groups

---