Draft: Advanced Search: Add extension to the main index

What does this MR do and why?

This is part of &7257

This MR adds the extension field to our main Advanced Search index (gitlab-production).

#347704 (closed)

Screenshots or screen recordings

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How to set up and validate locally

1. Enable Advanced Search if it's not configured
2. Check that you don't have the extension field in the main index mapping by executing

   curl -s http://localhost:9200/gitlab-development | jq -c 'paths | select(.[-1] == "extension")'

3. Checkout this MR's branch git checkout 347704-add-extension-field-to-es-mappings
4. In order to execute Advanced Search migrations execute this command in rails console:

   Elastic::MigrationWorker.new.perform

5. Ensure that this commands now returns results:

   curl -s http://localhost:9200/gitlab-development | jq -c 'paths | select(.[-1] == "extension")'

6. You can also inspect the output of http://localhost:9200/gitlab-development to find

   "extension": {
     "type": "keyword"
   }

7. Also, you can check log/elasticsearch.log and check that the migration has been successfully applied.

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 #347704 (closed)