Maps the integer raster values used by the SoilGrids 2.0
"MostProbable WRB" layer to soilKey's two-letter RSG codes (the
codes used in inst/rules/wrb2022/key.yaml).
Details
The numeric values follow the order used by ISRIC; users with a
different convention can override this via the lut argument
to spatial_prior_soilgrids.
