In Solr 9, the SOLR_HOME directory has moved to /var/solr/data and the actual index data lives inside /var/solr/data/data.
TODO:
Detail on how to delete old search indexes and import new ones. Existing search indexes produced by Solr 7 cannot be imported on Solr 9 (which only supports indexes produced by Solr 8 and 9).
TODO: