LogsDB data migration integration testing (elastic#112710) (elastic#113448)

Here we test reindexing logsdb indices, creating and restoring
snapshots. Note that logsdb uses synthetic source and restoring
source only snapshots fails due to missing _source.

(cherry picked from commit f7880ae)

Author: salvatore-campagna

docs/reference/snapshot-restore/repository-source-only.asciidoc

@@ -18,7 +18,7 @@ stream or index.
 ==================================================
 
 Source-only snapshots are only supported if the `_source` field is enabled and no source-filtering is applied.
-When you restore a source-only snapshot:
+As a result, indices adopting synthetic source cannot be restored. When you restore a source-only snapshot:
 
  * The restored index is read-only and can only serve `match_all` search or scroll requests to enable reindexing.