Update gnomAD resources to reflect free Google Cloud-hosted paths

[gtiao]

Prefixes should change from gs://gnomad-public/ or gs://gnomad-public-requester-pays/ to gs://gcp-public-data--gnomad

[nawatts]

Along with this, we should also remove the gnomad-public-requester-pays bucket from the getting started and VEP docs.

[nawatts]

#373 added support for reading resources from the Google Cloud Public Datasets bucket. Do we want to make that the default?

[gtiao]

Need to check that all recent, most updated files are synced to public GCP bucket, and then can proceed.

Nick notes that would be better/ideal to have entire bucket synced to GCP public bucket.

[nawatts]

A quick check revealed that the public GCP bucket's resources directory only contains the VEP 85 and 95 context Hail Tables.

So most of the reference data resources are not available from that bucket.

[nawatts]

We could still change the default source to the GCP bucket, if we assume that most users will be interested in the gnomAD resources vs the reference data resources.

#388 will also help with this by adding a more informative error message when a resource is not available from the selected source.