Add info about obsolete migrations to elastic rake tasks

What does this MR do and why?

Related to #413524 (closed)

We've run into a lot of issues where major upgrades are performed but Advanced Search migrations are not completed from the previous version. Currently the info that a migration is obsolete is available in logging only. This MR will provide customers and support team members a better view of the state of Advanced Search migrations.

The MR adds info about obsolete migrations to elastic rake tasks and update specs.

• gitlab:elastic:info added obsolete to list of pending migrations and to current migration info
• gitlab:elastic:list_pending_migrations added obsolete to list of pending migrations

Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. make sure gdk is setup for elasticsearch
2. advanced search is enabled/setup
3. manually delete a few migrations from the migrations index (note: this will require you to reindex everything from scratch)

   curl --request DELETE --url http://localhost:9200/gitlab-development-migrations/_doc/20201105181100 
   curl --request DELETE --url http://localhost:9200/gitlab-development-migrations/_doc/20230320011400 

4. bundle exec rake gitlab:elastic:info
5. bundle exect rake gitlab:elastic:list_pending_migrations

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.