Fix/compute effective date in partner

CHANGELOG:

### Fixed
- [#529](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/529) Change compute effective date method to get the date from the last paid share.

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After deploy this feature, we must execute the script added with https://gitlab.com/coopdevs/migration-tryton-odoo-somconnexio-scripts/-/merge_requests/14