Use BKG data to improve bounding boxes based on ARS/AGS

We currently use the ARS/AGS from Destatis which provide point geometry for each regional key. At https://gdz.bkg.bund.de/index.php/default/digitale-geodaten/verwaltungsgebiete/verwaltungsgebiete-1-250-000-stand-01-01-vg250-01-01.html, there is also a Shapefile available which provides polygons for the regions which we would important instead of the Excel file to provide better bounding boxes compared to those based on the points.

Technically, this would also be nice insofar we could drop the Calamine depedency if we do not need to read Excel any more.