Upgrade to Hibernate Search 6.1.8.Final

Starting with this version, we regularly test Hibernate Search in combination with Hibernate ORM 6.1 AND 6.2, so this should put us in a better situation should we decide to ship ORM 6.2.

Apart from that, it's mostly bugfixes.

https://in.relation.to/2023/01/31/hibernate-search-6-1-8-and-5-11-12/