[FIX] hr_holidays: write access for approvers

Without write permissions, Time Off Managers are unable to approve
nor refuse leave requests. It seems there was a mismatch with
`hr_holidays.group_hr_holidays_user` as "Approvers" and actual users
at some point.

Bug reported via p/feedback by CMO and LEM after v14.0 migration.

closes odoo/odoo#61943

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>