[FIX] hr_contract: Reset current contract on stage modification

Purpose
=======

Let's say we have 2 contracts A and B:

A --> Running
B --> Cancelled
contract_id : A

B is set to Running
Contract_id : B

B is set to Cancelled
contract_id : B

In this case, the current contract should be reset to A.

X-original-commit: f736264881cbb3086430ea8cd7dba7c4f5a351f4
Part-of: odoo/odoo#92040