IBAN not required in SR API if is sponsored

CHANGELOG:

### Changed
- [#668](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/668) IBAN not required in SR API if is sponsored

Related card: https://trello.com/c/806IY9hV/1043-iban-no-obligatoria-en-creacion-de-subscriptionrequest-si-es-apadrinado-o-cc

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