Enable concurrent_segment_search auto mode by default

Signed-off-by: Vikasht34 <viktari@amazon.com>

Author: Vikasht34

server/src/main/java/org/opensearch/search/SearchService.java

@@ -279,7 +279,7 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
 
     public static final Setting<String> CLUSTER_CONCURRENT_SEGMENT_SEARCH_MODE = Setting.simpleString(
         "search.concurrent_segment_search.mode",
-        CONCURRENT_SEGMENT_SEARCH_MODE_NONE,
+        CONCURRENT_SEGMENT_SEARCH_MODE_AUTO,
         value -> {
             switch (value) {
                 case CONCURRENT_SEGMENT_SEARCH_MODE_ALL:
@@ -298,7 +298,7 @@ public class SearchService extends AbstractLifecycleComponent implements IndexEv
     // settings to configure maximum slice created per search request using OS custom slice computation mechanism. Default lucene
     // mechanism will not be used if this setting is set with value > 0
     public static final String CONCURRENT_SEGMENT_SEARCH_TARGET_MAX_SLICE_COUNT_KEY = "search.concurrent.max_slice_count";
-    public static final int CONCURRENT_SEGMENT_SEARCH_TARGET_MAX_SLICE_COUNT_DEFAULT_VALUE = 0;
+    public static final int CONCURRENT_SEGMENT_SEARCH_TARGET_MAX_SLICE_COUNT_DEFAULT_VALUE = 2;
 
     // value == 0 means lucene slice computation will be used
     public static final Setting<Integer> CONCURRENT_SEGMENT_SEARCH_TARGET_MAX_SLICE_COUNT_SETTING = Setting.intSetting(