[FIX] hr_expense: fix the approval of multiple expense sheet

Steps to reproduce the bug:
- Go to Expenses > Expense Reports to approve
- Select expense reports with different managers
- Click on the Action drop-down button > Approve report

Problem:
Traceback is triggered because we try to access `"self.user_id"` (manager) of expenses as a single element,
While we have several `”user_id”`

Solution:
- Filter the `"hr.expense.sheet"` which must be approved
- Approve each expense separately

opw-2622837

closes odoo/odoo#75093

Signed-off-by: Djamel Touati <DjamelTouati@users.noreply.github.com>