Index namespaces traversal_ids in descending order

When we sort groups by descendants we perform a comparison of traversal_ids by their natural array ordering. Such as:

WHERE next_traversal_ids_sibling("superset"."traversal_ids") > "namespaces"."traversal_ids" 
      "superset"."traversal_ids" <= "namespaces"."traversal_ids" 

We already have a btree index for namespaces traversal_ids:

CREATE INDEX index_btree_namespaces_traversal_ids ON namespaces USING btree (traversal_ids);

However this index does not perform well in some cases. For example, here is a query with the current unordered index and here is the same query with the ordered index.

$ rails db:migrate:down:main VERSION=20220518043259                                                                                                                                                                == 20220518043259 IndexNamespacesTraversalIdsDesc: reverting ==================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :traversal_ids, {:name=>"index_btree_namespaces_traversal_ids_desc", :algorithm=>:concurrently})
   -> 0.0212s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- remove_index(:namespaces, {:name=>"index_btree_namespaces_traversal_ids_desc", :algorithm=>:concurrently, :column=>:traversal_ids})
   -> 0.0202s
-- execute("RESET statement_timeout")
   -> 0.0008s
== 20220518043259 IndexNamespacesTraversalIdsDesc: reverted (0.0581s) =========


$ rails db:migrate:up:main VERSION=20220518043259                                                                                                                                                                  == 20220518043259 IndexNamespacesTraversalIdsDesc: migrating ==================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :traversal_ids, {:order=>{:traversal_ids=>:desc}, :using=>:btree, :name=>"index_btree_namespaces_traversal_ids_desc", :algorithm=>:concurrently})
   -> 0.0186s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:namespaces, :traversal_ids, {:order=>{:traversal_ids=>:desc}, :using=>:btree, :name=>"index_btree_namespaces_traversal_ids_desc", :algorithm=>:concurrently})
   -> 0.0043s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20220518043259 IndexNamespacesTraversalIdsDesc: migrated (0.0325s) =========

Related to #350637 (closed)