[FIX] (purchase_)stock,mrp: add indexes for orderpoint_id

Because the "Replenishment" can unlink a lot (thousand of
`stock.warehouse.orderpoint`).
and psql needs to check every Many2one `orderpoint_id` constraint
and without index it takes too much time.

opw-2637321

closes odoo/odoo#76030

Signed-off-by: William Henrotin <Whenrow@users.noreply.github.com>