Deprecate and remove 'V2RequestSupport' and related methods

The V2RequestSupport interface (and associated methods) were created as a way to convert certain SolrRequest objects that supported it into their v2 counterparts. In practice though, this conversion logic was costly to write and maintain, so very few SolrRequest implementations ever supported it. At its peak, only a few "collection admin" APIs ever supported conversion, and these were removed a year or so back.

Instead, we've moved towards generating v2 SolrRequest implementations from our OpenAPI Spec ("OAS").

We should deprecate V2RequestSupport (and related methods) in 9.x and remove it entirely in 10.