Elasticsearch versioned schema for other ActiveRecord models

The CE port is at: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31660

What does this MR do?

A continuation of !14428 (merged), applying what has been done to Snippet to other ActiveRecord models.

For example, code is moved from IssuesSearch modules to Elastic::Latest::IssueClassProxy and Elastic::Latest::IssueInstanceProxy classes. The functionalities are not changed.

This is part of https://gitlab.com/gitlab-org/gitlab-ee/issues/10156 and https://gitlab.com/gitlab-org/gitlab-ee/issues/328.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Related to #328 (closed)