Remove duplicated ES administration Rake tasks

Summary

This is a follow-up to !17230 (closed) / !18254 (merged). Please resolve before merging !17230 (closed) into master

With the introduction of the new ES admin UI and the corresponding API, the Rake tasks become outdated or are obsolete.

I don't think there's much use in maintaining most of the old gitlab:elastic:* Rake tasks:

• gitlab:elastic:index
• gitlab:elastic:index_projects
• gitlab:elastic:create_empty_index / gitlab:elastic:recreate_index
• gitlab:elastic:delete_index

These were kept in !18254 (merged) for the moment because our QA test suite uses them to setup the ES index, but this could be replaced with calls to the API.

The tasks are also not very suitable for multi indices management. For example, create_empty_index and delete_index ideally are more applicable as a single-index operation, rather than multi-indices operation, but the rake tasks right now would affect all indices.

Other follow-up issues fill in the missing functionality provided by other Rake tasks:

• gitlab:elastic:index_projects_status: #38124
• gitlab:elastic:projects_not_indexed: #14621 (closed)
• gitlab:elastic:index_snippets: #13641 (closed)
• gitlab:elastic:create_empty_index, gitlab:elastic:clear_index_status: #11206

Some special-purpose tasks could be kept/removed, or implemented in the UI/API as well:

• gitlab:elastic:clear_locked_projects
• gitlab:elastic:test:index_size
• gitlab:elastic:test:index_size_change

Improvements

Remove the Rake tasks that are duplicated in functionality.

Risks

Users might prefer having Rake tasks for some tasks, this would need to be determined.

If we want to keep the creation/deletion tasks, we'd need to extend them to properly support multiple indices.

Involved components

• ee/lib/tasks/gitlab/elastic.rake
• ee/lib/tasks/gitlab/elastic/test.rake
• doc/integration/elasticsearch.md