Update default VO Cone Search URLs for "classic" API

**Documented Usage**

For the “classic” API ported from Astropy, the default Cone Search services used are a subset of those found in the STScI VAO Registry. They were hand-picked to represent commonly used catalogs below:

* 2MASS All-Sky
* HST Guide Star Catalog (also default for “new” Astroquery-style API)
* SDSS Data Release 7
* SDSS-III Data Release 8
* USNO A1
* USNO A2
* USNO B1

**The Problem**

Some of the hand picked services have become unstable over time. Also, SDSS Data Release now in up to 13.

**The Solution**

Someone needs to dive into STScI VAO registry for Cone Search services (over 10k of them) and hand-pick a whole new and updated set of services. They don't need to be exact replacement, but should be selected based on their merit for general Astronomy community (i.e., what are people using Cone Search for and which would be the most useful services?).

*OR*

A more drastic change is to deprecate this feature completely and force users to provide their own list.

**Priority**

The priority is low, because user can always provide their own favorite Cone Search services, which is documented.

Start by reviewing the classic API's documented default Cone Search services and the STScI VAO Registry. The issue names no repository files or tests, so first locate where these defaults and their documentation are defined. Done means selecting a stable, useful updated service set or deciding to deprecate the defaults and requiring users to provide their own list.

Written by the indexing model from the issue text.