Replace most uses of GenericSolrRequest in Solr code

GenericSolrRequest, a SolrRequest implementation that can be used to make requests to arbitrary endpoints via a specified HTTP method and path, is used in 50-60 places through Solr's codebase.

In most of these places, it's used as a "last resort" because SolrJ doesn't offer a more specific SolrRequest implementation that covers the API the developer wanted to use.

Where this is the case, we should fill these gaps in SolrJ with new SolrRequest implementations and remove the usage of GSR.