TEST: decrease logging level in the flush test

Relates #31629

Author: dnhatn

server/src/test/java/org/elasticsearch/index/shard/IndexShardIT.java

@@ -400,7 +400,7 @@ public void testMaybeRollTranslogGeneration() throws Exception {
         }
     }
 
-    @TestLogging("_root:DEBUG,org.elasticsearch.index.shard:TRACE,org.elasticsearch.index.engine:TRACE")
+    @TestLogging("org.elasticsearch.index.shard:TRACE,org.elasticsearch.index.engine:TRACE")
     public void testStressMaybeFlushOrRollTranslogGeneration() throws Exception {
         createIndex("test");
         ensureGreen();