Resolve "Add schema_version in the commit index"

What does this MR do and why?

This MR will add an integer field schema_version in the commits index mapping. This attribute can be used to determine completed? in case of full reindexing of the commits index.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Open the rails console

   bundle exec rails c

2. Run the following commands

require_relative 'ee/elastic/migrate/20230613555555_add_schema_version_to_commits.rb' 
AddSchemaVersionToCommits.new(20230613555555).migrate

3. Check the mapping of the commits index

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

4. Make sure there is a field schema_version with type as integer is present

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 #414804 (closed)