MAINT: cleanup configitem defaultvalues

The discussion in https://github.com/astropy/astroquery/pull/3413 made me wonder if we need to review the configs for each and every module and see if the ConfigItem defaultvalues are used correctly, namely if it's a list it won't be possible to override at run-time.

And that behaviour is IMO disadvantageous in each case when we don't deal with mirrors and multiple servers, for example in the case of IRSA we routinely test server side changes by runtime changing the server url to the test server, and I see no reason to ship that url as a possibility in the list.

cc @keflavich for opinion and as he may remember why we kept using lists for non-mirrored services.

Start by reading the discussion in pull request 3413 and reviewing ConfigItem defaultvalues across each astroquery module. Identify list defaults for mirrored services versus single-server URLs, then verify that the intended runtime configuration behavior is consistent. Done means every module has been reviewed and the non-mirror cases no longer prevent runtime URL overrides.

Written by the indexing model from the issue text.