The xwiki-solr9-* packages don't work out of the box with standard Solr 9 install

The change is that Solr 9 comes with a new security policy enabled by default (it actually existed in Solr 8 too, but it was disabled by default) which does not include the location of those files, so it cannot load the XWiki cores.

On Debian (and probably other Linux distributions), the easiest workaround to go back to Solr 8 behavior is to disable SOLR_SECURITY_MANAGER_ENABLED in /etc/default/solr.in.sh.