Organize soft_deletes settings into a single group

GetSettingsResponseTests fails on the ccr branch because it excludes the
root-level settings while we introduced "index.soft_deletes" - a
root-level setting in this branch.

This commit moves "index.soft_deletes" to the grouped-level settings.
Moreover, as we are going to introduce more settings for soft_deletes, I
think it's better to organize all these settings into a single group.

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

@@ -240,7 +240,8 @@ public final class IndexSettings {
     /**
      * Specifies if the index should use soft-delete instead of hard-delete for update/delete operations.
      */
-    public static final Setting<Boolean> INDEX_SOFT_DELETES_SETTING = Setting.boolSetting("index.soft_deletes", true, Property.IndexScope);
+    public static final Setting<Boolean> INDEX_SOFT_DELETES_SETTING =
+        Setting.boolSetting("index.soft_deletes.enabled", true, Property.IndexScope);
 
     /**
      * Controls how many soft-deleted documents will be kept around before being merged away. Keeping more deleted