Item properties `created` and `updated` do not align with the STAC specification

[pjhartzell]

created and updated Item property fields are populated with values that appear to be a data processing date:

        created = datetime.datetime.fromisoformat(
            metadata.find_text_or_throw(
                "ECSDataGranule/ProductionDateTime", missing_element("created")
            )
        )
        updated = datetime.datetime.fromisoformat(
            metadata.find_text_or_throw("LastUpdate", missing_element("updated"))
        )

Per the STAC spec, these datetimes should reflect when the Item was created and updated, not when the data was created/processed/updated.

I think the created and updated fields need to be removed from the Items.