Fix enum scaffolding for PG < 9.1 (#2162)

Fixes #2160

Author: roji

src/EFCore.PG/Scaffolding/Internal/NpgsqlDatabaseModelFactory.cs

@@ -986,11 +986,11 @@ AND NOT EXISTS (SELECT * FROM pg_depend AS dep WHERE dep.objid = cls.oid AND dep
     /// </summary>
     private static HashSet<string> GetEnums(NpgsqlConnection connection, DatabaseModel databaseModel)
     {
-        const string commandText = @"
+        var commandText = $@"
 SELECT
   nspname,
   typname,
-  array_agg(enumlabel ORDER BY enumsortorder) AS labels
+  array_agg(enumlabel{(connection.PostgreSqlVersion >= new Version(9, 1) ? " ORDER BY enumsortorder" : "")}) AS labels
 FROM pg_enum
 JOIN pg_type ON pg_type.oid = enumtypid
 JOIN pg_namespace ON pg_namespace.oid = pg_type.typnamespace