[FIX] hr_holidays: operator of field "holiday_status_id" domain

This replaces the former domain operator '?=' which is not allowed, This domain property, new to Odoo 16 does not permit the edition of the field with Studio, this change will enhance the configuration and will prevent future issues.

owp-3484144

closes odoo/odoo#135243

Signed-off-by: Sofie Gvaladze (sgv) <sgv@odoo.com>