projection error in to_xarray

ItemCollection.to_xarray calls stackstac which is not compatible with STAC extension projection v2 and leads to an error, see stackstac issue #262.

Until a new version of stackstac solving this issue (i.e. stackstac > v0.5.1), projection v1.2 should be use, i.e. adding "proj:epsg" where "proj:code" is available.