[FIX] sale_stock: reservation not updating in picking when increasing SO

When increasing the quantity of a validated Sale Order, the reserved
quantity in the related pickings aren't updated to match the new
quantity set in the Sale Order. It requires to either let the scheduler
run or clicking the 'Check Availability' button to update the reserved
quantity in those pickings.

If the reservation_method for this picking_type is set to 'at_confirm',
this is not the intended behavior as this picking is already confirmed,
so it should be updated without user input.

The reason is that when the new moves (which adds to the quantity) is
merged to the old one, its status is changed to 'partially_available'
because at that moment, the product_qty is bigger than the
reserved_quantity, which needs to be updated to match the new needs.

closes odoo/odoo#76752

closes odoo/odoo#78070

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