An embedded Solr search core created in 16.6.0 or 16.7.0 won't work anymore in 16.8.0+

The schema changed in 16.6.0 (but by default this does not cause the core to be re-created) and since 16.8.0 it requires the lib/ folder to contain a special JAR which was not here before.

The workaround is to stop XWiki, delete <permdir>/store/solr/search_9} and restart.