[17.0][FW] hr_holidays_public: Deprecate use of employee_id in favour of partner_id

[grindtildeath]

Helper functions to find public holidays did allow to use an hr.employee to filter country and states based on the employee address.

Since only the address of the employee was used, modifying the functions to use a res.partner instead of an hr.employee allows more possibilities such as checking public holidays for customers and suppliers.

Forward port of:

• [x] #110

[github-actions[bot]]

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