Which is the highest version of Solr that is compatible with the "Apache Solr Search" module? I have a setup of Drupal 7 with the Apache Solr Search module. I am planning to upgrade the solr server's current installed version(3.6) to a higher version. 

The latest version of Solr available in their site is 8.5.1. 

Which is the highest version of Solr that is compatible with the 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.