[Deprecated] Continue to improve performance of Merge Request Changes API under load into S3 tier

With https://gitlab.com/gitlab-org/gitlab/-/issues/225322, performance of the Merge Request Changes API endpoint was improved significantly by approximately 63% from 7.9s to around 2.9s (can fluctuate between 2.1s to 2.9s): ``` * Environment: 10k * Environment Version: 13.9.0-pre `adef80adead` * Option: 60s_200rps * Date: 2021-02-18 * Run Time: 1h 18m 27.86s (Start: 04:35:52 UTC, End: 05:54:20 UTC) * GPT Version: v2.6.0 ❯ Overall Results Score: 98.07% NAME | RPS | RPS RESULT | TTFB AVG | TTFB P90 | REQ STATUS | RESULT ---------------------------------------------------------|-------|----------------------|-----------|----------------------|----------------|------- api_v4_projects_merge_requests_merge_request_changes | 200/s | 131.14/s (>16.00/s) | 1385.11ms | 2078.09ms (<12000ms) | 99.97% (>99%) | Passed ```  While this is still in the ~"severity::2" [performance targets tier](https://about.gitlab.com/handbook/engineering/quality/issue-triage/#severity) that piece of work was significant and for further improvements more separate pieces of work need to be explored so a new issue is being opened to track this. Of note compared to the previous issue the endpoint is showing a heavier strain on the GitLab Rails nodes (CPU specifically) Task is to continue this great work and improve performance further into the ~"severity::3" tier (<2000ms). ### Deprecated The [changes API endpoint](https://docs.gitlab.com/ee/api/merge_requests.html#get-single-merge-request-changes) has been deprecated and will not be worked on. It'll be removed in API v5. A new [diffs API endpoint](https://docs.gitlab.com/ee/api/merge_requests.html#list-merge-request-diffs) has been introduced which supports pagination to replace it.

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