Fix GraphQL API mergeUser is null when mergeWhenPipelineSucceeds is false

What does this MR do and why?

Solves #321477 (closed)

GraphQL field mergeUser is described in documentation as User who merged this merge request.. However, it is only the user who set MR to Merge When Pipeline Succeeds. For merge request that merged without MWPS, it is nil.

This MR fixes that, so that this field is actually the user who merged the MR.

It will still return the user as soon as MWPS is set, for backwards compatibility. I've also clarified that fact in documentation.

The issue was that merge_request.merge_user is actually only the user that set MWPS. When fetching the merged user, merge_request.metrics&.merged_by should be used instead. For example, here it is used in REST API: https://gitlab.com/gitlab-org/gitlab/-/blob/6b591ca23013381a7c4a84f47691b91216911811/lib/api/entities/merge_request_basic.rb#L7

Alternative fix would be to keep mergeUser as it is, and add a separate field, like mergedBy, but I think that it would be a less intuitive option.

Screenshots or screen recordings

Before:

After:

How to set up and validate locally

1. Create two MRs in a project with a pipeline.
2. Merge one normally, and one by setting it to MWPS before the pipeline succeeds.
3. Use following GraphQL query, after changing fullPath to your test project:

{
  project(fullPath: "flightjs/Flight") {
    name,
    mergeRequests {
    	nodes {
        id,
        iid,
        title,
        mergeWhenPipelineSucceeds,
        mergeUser {
          name
        }
      }
    }
  }
}

After my changes both MR should have valid mergeUser set. Before, only the MR merged using MWPS would have non-null mergeUser.

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