Reduce stateless default refresh to 5s

Having done recent optimizations, we can now start reducing the
refresh interval for stateless. As a first step we will reduce
the default to 5s.

Relates ES-7774

server/src/main/java/org/elasticsearch/index/IndexSettings.java

@@ -284,7 +284,7 @@ public final class IndexSettings {
         TimeValue.MINUS_ONE,
         Property.NodeScope
     ); // TODO: remove setting
-    public static TimeValue STATELESS_DEFAULT_REFRESH_INTERVAL = TimeValue.timeValueSeconds(15); // TODO: this value is still not final
+    public static TimeValue STATELESS_DEFAULT_REFRESH_INTERVAL = TimeValue.timeValueSeconds(5); // TODO: this value is still not final
     public static TimeValue STATELESS_MIN_NON_FAST_REFRESH_INTERVAL = TimeValue.timeValueSeconds(5);
     public static final Setting<TimeValue> INDEX_REFRESH_INTERVAL_SETTING = Setting.timeSetting("index.refresh_interval", (settings) -> {
         if (EXISTING_SHARDS_ALLOCATOR_SETTING.get(settings).equals("stateless") && INDEX_FAST_REFRESH_SETTING.get(settings) == false) {