[FIX] l10n_in_sale: only take GSTIN from shipping address if shipping_gstin is set

When there is a bill to ship to and ship to is the third party then GSTIN is set from customer address(partner_id), not from shipping address(partner_shipping_id)

closes odoo/odoo#73684

Signed-off-by: oco-odoo <oco-odoo@users.noreply.github.com>