Fix ConcurrentSnapshotsIT.testMasterFailoverDuringStaleIndicesCleanup

As a result of elastic#86514 we now
remove the snapshot delete from the cluster state before cleaning up
stale shard level blobs so we have to block master earlier on deleting the
root level index-N blob to get the desired cluster state where delete
and snapshot creation are both in the cluster state at the same time.

closes elastic#88061

server/src/internalClusterTest/java/org/elasticsearch/snapshots/ConcurrentSnapshotsIT.java

@@ -1076,7 +1076,7 @@ public void testMasterFailoverDuringStaleIndicesCleanup() throws Exception {
 
         final List<String> fullSnapshotsToDelete = createNSnapshots(repoName, randomIntBetween(1, 5));
         final String masterName = internalCluster().getMasterName();
-        blockMasterOnAnyDataFile(repoName);
+        blockMasterFromDeletingIndexNFile(repoName);
         final ActionFuture<AcknowledgedResponse> deleteAllSnapshotsWithIndex = startDeleteSnapshots(
             repoName,
             fullSnapshotsToDelete,