[FIX] project: prevent creation of private tasks for others

Before this commit, the current user can create a private task for
another user.

This commit fixes the access rights to avoid the user to create private
tasks for another user.

closes odoo/odoo#90938

X-original-commit: 106826b1ffcd9a18ce5d4674c526bce4f2efca9e
Signed-off-by: Laurent Stukkens (ltu) <ltu@odoo.com>
Signed-off-by: Xavier <xbo@odoo.com>