The Solr cores are looked up using the major version of the remote Solr server instead of the one they are designed for

The Solr cores used by XWiki are designed for a given major version of Solr (the one XWiki is bundling, currently 9), and named accordingly (xwiki_search_9, etc.). But when using a remote Solr server, the cores are looked up using the major version of that server instead.

With a remote Solr 10 server:

• XWiki looks for xwiki_search_10 instead of xwiki_search_9.
• The fallback on the cores designed for the previous major version (added in XWIKI-24326) looks for xwiki_search_10 again, since the suffix of the core name is always computed from the server version, whatever the requested version.
• So XWiki does not find any of the cores created by the administrator (with the expected _9 suffix), and the search does not work.
• And when a _10 core exists, it's considered as designed for Solr 10, and XWiki tries to migrate it from the "previous" core, which is resolved to the same _10 core.

The suffix problem also affects the embedded Solr: when XWiki moves to Solr 10, the migration of the cores from Solr 9 would look for the previous _9 core under the _10 name, and the data of the non-cache cores (events, ratings, extension index) would not be migrated.