Improve Elasticsearch migration helpers

Description

There are different ideas to improve the Elasticsearch migration helpers:

• Fix Rubocop offenses, including them under bounded contexts
• Create a ReindexTask helper. Idea from !182255 (comment 2368067051).
• Extract a shared example spec to remove a field from an index. Idea from !182551 (merged).
• Include testing examples in the documentation if there are shared examples, or link to merge requests.

Reference: https://docs.gitlab.com/development/search/advanced_search_migration_styleguide/#migration-helpers