[VSA] Display MRs instead of CI Builds for Test and Staging stages

Background

In group-level VSA, the default test and staging stages are unique because they display CI Build records, whereas other stages show either Issue or MR records. I'm fairly sure displaying CI Build records is no longer correct (legacy).

Problems to solve

• Performance. In group-level VSA, the default test and staging stages are timing out when listing the CI Build records. (Previously, it was hard to notice due to a frontend bug: #334487 (closed).) This is probably because the test and staging queries join on the ci_builds table and show build records on the UI. This makes the database scan way too many rows, which leads to statement timeout.
• Confusing UI. A single MR will produce multiple CI Build records if there are multiple jobs in the pipeline (causing excessive granularity in the UI) or the pipeline is run more than once (causing duplication in the UI).

Proposal

To improve the UI, show MR records (instead of CI Build records) for the test and staging stages. To improve the performance, eliminate the JOIN ON ci_builds because it will no longer be necessary.

Notes:

1. No change needed for the summary calculations (like median). The pipeline times are not used for those calculations. We use the data we have in the merge_request_metrics table: latest_build_started_at, latest_build_finished_at and first_deployed_to_production_at columns.
2. Removing the joins would also help the ongoing CI sharding effort.

Example of CI Builds