doc: clarify STAC version in first bullet list

[pjonsson]

The release notes for 1.12.0 state "STAC v1.1.0 is now the default" and the documentation at https://pystac.readthedocs.io/en/latest/ states in the first bullet:

Reading and writing STAC version 1.0. Future versions will read older versions of STAC, but always write the latest supported version. See STAC Spec Version Support for details.

I know the details are explained if you follow the link, but I think it would be nice if the bullet itself explicitly mentioned that both version 1.0 and 1.1.0 are supported.