Adress review comments

ee/app/models/ee/epic.rb

@@ -169,7 +169,7 @@ module EE
       # epic2 - parent: epic1
       # Returns: 2
       def deepest_relationship_level
-        return unless ::Gitlab::Database.postgresql?
+        return unless ::Group.supports_nested_objects?
 
         result =
           ActiveRecord::Base.connection.execute(

ee/lib/ee/gitlab/usage_data.rb

@@ -106,7 +106,7 @@ module EE
         }).merge(service_desk_counts).merge(security_products_usage)
 
         # MySql does not support recursive queries so we can't retrieve epics relationship depth
-        if ::Gitlab::Database.postgresql?
+        if ::Group.supports_nested_objects?
           usage_data[:counts] = usage_data[:counts].merge(epics_deepest_relationship_level)
         end