[FIX] stock_dropshipping: singleton in dropshipping rules

When you want to obtain `dropship_picking_type`, it is possible to have
several results. This will lead to an error when doing
`dropship_picking_type.id`.

```
dropship_picking_type = self.env['stock.picking.type'].search([
    ('company_id', '=', company.id),
    ('default_location_src_id.usage', '=', 'supplier'),
    ('default_location_dest_id.usage', '=', 'customer'),
])
```
The fix ensures that `dropship_picking_type` contains only one element.

closes odoo/odoo#61256

X-original-commit: 3e3b1b335ced1ff20129172e6a7b2b0dcbe1a20d
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>

addons/stock_dropshipping/models/res_company.py

@@ -89,7 +89,7 @@ class ResCompany(models.Model):
                 ('company_id', '=', company.id),
                 ('default_location_src_id.usage', '=', 'supplier'),
                 ('default_location_dest_id.usage', '=', 'customer'),
-            ])
+            ], limit=1, order='sequence')
             dropship_vals.append({
                 'name': '%s → %s' % (supplier_location.name, customer_location.name),
                 'action': 'buy',