Advanced Search: Fix AddUpvotesToMergeRequests migration

What does this MR do and why?

This MR fixes an issue with the AddUpvotesToMergeRequests migration, which adds new Elasticsearch mappings and backfills the field in one step. This MR adds a new migration that only adds mappings and puts it before AddUpvotesToMergeRequests so it always runs first.

Additional context: #341133 (closed)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

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.

Closes #341133 (closed)