[IMP] crm: take highest priority when merging leads

When merging leads final priority is currently the one of the lead considered
as the master lead. This is based on ``_sort_by_confidence_level`` who sorts
on confidence but not on priority.

We think final priority should be the highest one from merged leads. Indeed
if a top priority lead is merged into a low priority one we should not loose
the priority information. It is better to keep the final lead with an high
priority and let salesmen move it downwards.

LINKS

Task ID-2446759 (lead merge priority field management)
Task ID-2446883 (query counters fix)
PR odoo/odoo#65016