Move the validation to a contrains method

CHANGELOG:

### Fixed
- [#769](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/769) Validate VAT also in modifications.

Related card: https://trello.com/c/vKqIkLXz/1194-validaci%C3%B3-dni-duplicat-a-sr-i-a-creacio-manual-de-contactes

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