[FIX] hr_recruitment: newly hired

The current 'Newly Hired' filter is incorrect in several ways:
- the compute method doesn't assign the value correctly
- the search method mixes `hr.applicant` IDs with `hr.employee` IDs

Moreover, it relies on the fact that the job position is closed. Some
positions might always be opened, so the result is inaccurate.

We make the simpler assumption that the newly hired people are employees
created less than 90 days ago.

opw-2125861

closes odoo/odoo#40889

X-original-commit: ee928babbe433285883f1e878d83392d7199a37d
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>