Rename to rustac

[gadomski]

As proposed in #641, we've renamed this repo to stac-utils/rustac. The only crate name change is stac-cli, which has been renamed to rustac. We haven't published a new version of the rustac crate, as we're currently mid-flight on some dependency upgrades that require us to patch geoarrow and cql2. Once those patches are removed, we will release a new (first) version of rustac.