Update token issuer/scope for new IGWN issuer

All GWDataFind server instances, including datafind.ligo.org need to be updated to accept tokens from the new issuer https://cilogon.org/igwn with the new read scope gwdatafind.read.

Given that most users aren't using tokens yet, we can probably do a step change, rather than supporting both old and new tokens at the same time.