[FIX] maintenance: `MemoryError` on compute

When computing the todo fields of maintenance requests, we could get a
`MemoryError` since we are fetching more fields that we actually need.

Here we rework the way the compute is done in order to avoid too much
prefetch.

closes odoo/odoo#82618

Signed-off-by: Christophe Simonis <chs@odoo.com>