Use explicitly tracked deployments for MR widgets

What does this MR do?

When displaying the deployments of a merge request in the MR widget, we used to rely on the latest CI pipeline to determine what the deployment was. In this commit we add the ability to use the table "deployment_merge_requests" to display the deployments of a merge request. This way the data displayed remains the same, regardless of the state of any CI pipelines.

These changes are currently hidden behind the "deployment_merge_requests_widget" project feature flag. We do not reuse the existing deployments feature flag, as we still want to record merge requests even if the UI is not able to display the data properly.

The plan is to have this running on GitLab.com for a while to see how the system behaves with this new setup. Once we are confident we will remove the feature flag and make this the default behaviour. The merge request that removes the feature flag will also add the changelog entry.

See #35741 for more information.

Issue to remove the feature flag later on: #36498 (closed)

Screenshots

Technically this MR does not make any changes to the UI, as it just changes where the UI gets its data from. Nevertheless, this is what it looks like to have a merge request display deployments:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.