remove stac_version and stac_extensions have been removed from ItemCollection as of 1.0.0-beta.3

[philvarner]

Describe the bug

As of STAC API 1.0.0-beta3 https://github.com/radiantearth/stac-api-spec/blob/master/CHANGELOG.md#v100-beta3---2021-08-06 ItemCollection no longer has fields:

stacVersion: StacVersion,
stacExtensions: List[String]

So these fields can be removed from the model.