Move Elastic to Search::Elastic namespace

Everyone can contribute. Help move this issue forward while earning points, leveling up and collecting rewards.

• Work on this issue
• Close this issue

There are several parts of the codebase that are namespaced as Elastic and not within the Search::Elastic. We need to review the items in https://gitlab.com/gitlab-org/gitlab/-/blob/master/.rubocop_todo/search/namespaced_class.yml, remove them one by one and move them Elastic -> Search::Elastic.