[FIX] stock: avoid write on move when create return line

When creating a wizard `stock.return.picking` and call
`_onchange_picking_id`, the creation of return picking line will perform
a write on move linked with it, due to `uom_id` being set as related with
`readonly=False`. Set `readonly` to `True` to avoid it

closes odoo/odoo#77187

X-original-commit: a0e6bc87
Signed-off-by: Tiffany Chang <tic@odoo.com>