Spatial extent invalid for mtbs

[m-mohr]

https://planetarycomputer.microsoft.com/api/stac/v1/collections/mtbs lists two bboxes, but according to the STAC Collection spec you must list three bboxes. First the union and then all sub-bboxes.

This leads to downstream issues in the clients where the first bbox is omitted: https://mspc.lutana.de/collections/mtbs