`merge_status` in Merge Request API does not reflect true status
**NOTE:** This issue is currently blocked by https://gitlab.com/groups/gitlab-org/-/epics/5598 which seeks to improve mergeability checks making it easier to diagnose and remedy these issues.
### Summary
The `merge_status` field in the Merge Request JSON API does indicate proper status if the merge is blocked due to a bad build.
### Steps to reproduce
1. Configure a project to disallow merging if build fails
2. Create a merge request for a branch whose tests fail
3. Use API to get info on that merge request
### Expected behavior
`merge_status: 'cannot_be_merged'` in JSON response
### Actual behavior
`merge_status: 'can_be_merged'` in JSON response
### Output of checks
#### Results of GitLab application Check
Unfortunately I don't have access to the terminal where our Gitlab server is running, but the Admin console indicates healthy. We're running v8.9.6.
#### Results of GitLab environment info
Ditto
issue