Solr backend should not call SearchStax blocked APIs

The SearchStax Solr servers do not allow access to some of the Solr admin endpoints, like admin/system and admin/luke.
The SearchStax-specific connector plugin included in the SearchStax Search API Module → and ported to this module in 📌 Add support for Token Authentication to this module Active therefore bypasses requests to these endpoints to avoid showing error messages to the user. We should make sure this still works in this module and also implement the same workarounds for Solr servers using other connector plugins when hooked to SearchStax servers.