Which is the highest version of Solr that is compatible with the "Apache Solr Search" module?
Given there is an issue in the project issue queue (Solr 7 integration) about allowing to use the module with Solr 7, the module is at the moment only compatible with Solr 5 and eventually Solr 6.

Notice that the README.txt file has not been updated since 6 years ago and that the solr-conf directory contains configuration files for Solr 1.4, 3.x, 4.x, and 5.x. This means the module has been tested with Solr 5.x, at least the 5.x version that was available 4 years ago.