internal/target: sort public_id descending

The previous sort would run the risk of missing items which
collide on update or create times because of the compound
comparison using strictly less than for both the timestamp
and the public_id. Sorting both descending avoids thi issue.

internal/db/schema/migrations/oss/postgres/80/02_target_base_table_updates.up.sql

@@ -36,9 +36,9 @@ begin;
 
   -- Add new indexes for the update time queries.
   create index target_create_time_public_id_idx
-      on target (create_time desc, public_id asc);
+      on target (create_time desc, public_id desc);
   create index target_update_time_public_id_idx
-      on target (update_time desc, public_id asc);
+      on target (update_time desc, public_id desc);
 
   analyze target;