Lucene/Solr 9; Java 17; Tomcat 10

[jan-niestadt]

Thanks to @eduarddrenth for this branch, which updates our previous experiment and solves more issues.

CURRENT STATUS: working, experimental. Will probably be merged in after releasing v4 soon

Old comments:

Main issue now seems to be that indexes created with Lucene 8 cannot be read by this Lucene 9 version. Normally, Lucene 9 should not have any issue reading Lucene 8 indexes, but our custom Codec probably causes issues with this. You can see this when running the tests, some of which run against a precreated (with Lucene 8) test index.