[Elasticsearch] Decouple code for versioned schema for repository and Wiki

A continuation of https://gitlab.com/gitlab-org/gitlab-ee/issues/328

Problem to solve

After converting ActiveModel related search modules in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14428, we need to

• convert RepositoriesSearch and WikiRepositoriesSearch so they can support multiple version of indices.
• allow the go indexer gitlab-elasticsearch-indexer to also support multiple version of indices

What does success look like, and how can we measure that?

Repository content indexing can support multiple indices.