In all previous Alfresco Content Services versions, Solr.war was bundled with the repository. With Alfresco Content Services 5.2.1, you no longer deploy a Solr.war to your application server. Solr 6 is an independently executable standalone application powered by a Jetty server.
Alfresco Content Services uses Solr 4 as the default search service index. For an improved and efficient search functionality, you can upgrade to Alfresco Search Services with Solr 6.