Enforce usage of Diff index

[kwin]

The new index definition format outlined at https://experienceleague.adobe.com/en/docs/experience-manager-cloud-service/content/operations/indexing#simplified-index-management-using-the-diff-index and https://jackrabbit.apache.org/oak/docs/query/indexing.html#diff-indexes is simpler to manage.

Also according to Adobe

Beginning September 2026, the old-style index management (full-copy naming convention) will be retired. Existing indexes that have already been deployed will continue to function, but you will no longer be able to deploy new versions using the old naming convention.

Therefore there should be a rule enforcing that. Seems this is also recommended for completely custom indexes.

[kwin]

There is further hints at https://issues.apache.org/jira/browse/OAK-12010. Also there is some limitation on support of /apps and /libs indexing (compare with Netcentric/accesscontroltool#893), so right now it is unclear whether both index definition formats stay valid.