Major Description
In order to support scenarios leveraging Solr as an Oak index, it is recommended to upgrade to a supported version.
Having Solr 8 being released just recently, we could upgrade to that or to 7.x, however I suggest to upgrade to Solr 6.6.6 which would require a more flawless update for people using 5.5.5, because 6.x is back compatible with 5.x.