fix: contract holder change - new product category
CHANGELOG:
### Fixed
- [#994](https://git.coopdevs.org/coopdevs/som-connexio/odoo-somconnexio/-/merge_requests/994) Fix contract holder change available products: filter by old product category
Related card: https://trello.com/c/3Z2lTqYw/1547-wizard-canvi-de-titular-filtri-productes-correctament