[FIX] l10n_lu: fix tax report lines' codes for XML export 2.0

When exporting the tax report's monthly xml with enterprise, the file wasn't accepted by the government in case 0% taxes were used.

Grid 033 is optional, and can be used for custom tax rates. If specified, it MUST always go with grid 403 (giving the tax rate) and 042 (giving the tax amount). In Odoo, we made the choice to use it for 0% taxes, so 403 and 42 will always be present, and always 0.

In community, grids number were missing on the lines' labels, and the codes (used to retrieve the value of each grid when generating the XML) were not correct.

closes odoo/odoo#70744

Related: odoo/enterprise#18287
Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>