Add hidden attribute in merge_requests index

What does this MR do and why?

This MR will add a boolean field hidden in the MergeRequest ES mapping. The value of hidden will be true if the author of the MR is banned and the value will be false if the author of the MR is unbanned. This field can be used to filter out the hidden MRs in the search results. This is a reference MR done to fix the issues:

Screenshots or screen recordings

How to set up and validate locally

1. Run local development

gdk start

2. Run the elasticsearch migration

gitlab:elastic:index

3. Run the ES _search query and make sure the hidden attribute is present

curl -H 'Content-type: application/json' -XGET 'http://localhost:9200/gitlab-development-merge_requests/_search' | json_pp

4. Now ban an author of an issue, and run the _search query again. Set the author_id to the banned user_id.

curl -H 'Content-type: application/json' -XGET 'http://localhost:9200/gitlab-development-merge_requests/_search' -d '{ "query": { "term": { "author_id":  47 } } }' | json_pp

5. Make sure the hidden attribute value is true. You might have to wait for a few seconds to finish the Sidekiq
6. Now unban the user and run the same query

curl -H 'Content-type: application/json' -XGET 'http://localhost:9200/gitlab-development-merge_requests/_search' -d '{ "query": { "term": { "author_id":  47 } } }' | json_pp

7. Make sure the hidden attribute value is false. You might have to wait for a few seconds to finish the Sidekiq job Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #394746