Bump swh.objstorage requirement

We now depend on swh.objstorage.interface.objid_from_dict

requirements-swh.txt

 swh.core[db,http] >= 3.6.1
 swh.model >= 6.13.0
-swh.objstorage >= 0.2.2
+swh.objstorage >= 2.3.1
 swh.storage >= 2.0.0
 swh.journal >= 0.1.0