[FIX] hr_contract: Display the same count than the number of related contract

Purpose
=======

We open the contract history view, the count shouldn't be based on the
contracts, but on the history itself (to exclude canceled contracts for
example).

closes odoo/odoo#92040

X-original-commit: 5754eb95d681ec9ad63bf1a3c98a60cbb75d2869
Related: odoo/enterprise#27652
Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>