Update MAST registry link in vo-conesearch validator

[tsdower]

MAST's TAP service, including for the IVOA registry, has moved to a new (Python!) platform with a new URL. While we are providing limited forwarding of sync mode queries, updating links in clients provides future proofing, spares network hops, and is less confusing.

https://github.com/astropy/astroquery/blob/main/astroquery/vo_conesearch/validator/__init__.py#L17
https://vao.stsci.edu/regtap/tapservice.aspx/ should be replaced with https://mast.stsci.edu/vo-tap/api/v0.1/registry/, all arguments remain the same.

I can do the one-line PR.

[bsipocz]

Yes please, a PR would be nice!

[bsipocz]

Also we have been talking for a while about removing that module, but that's not happening in a useful enough timescale for this issue)