Resolve "Deployments->Environments displays wrong commit refs/tags"

What does this MR do and why?

This MR updates environment.last_deployment to return the deployment that was finished last, instead of the deployment with the highest id.

Customer Impact

After investigation on a read-only production replica of gitlab.com with the following script

Environment.all.each_batch(of: 1000) do |batch|
    batch.each do |e|
        if e.deployments.success.count && e.deployments.success.ordered.first&.id != e.deployments.success.distinct_on_environment.first&.id
            success_changed_envs.push(e.id)
        end
    end
end
puts "Success Changes #{success_changed_envs.count}"

I found 6 last_deployment that would be different.

• 2 had nil for finished_at
• The other 4 had finished_at time differences of 0.243551, 35.376646, 39.283417 and 710.027255 seconds.

This seems like this change has fairly low customer impact and in each case, changes last_deployment to the actually last deployment instead of just the latest id

Performance Impact

I found the environment with the highest deployment count on gitlab.com (11115 deployments) then compared the old vs new SQL statement performance on postgres.ai

Old (success.distinct_on_environment): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9464/commands/33572

New (success.ordered): https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/9464/commands/33573

The new method is a slight performance improvement (~15.7ms to ~4.7ms)

Screenshots or screen recordings

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How to set up and validate locally

From the Rails Console

1. Feature.enable(:env_last_deployment_by_finished_at)
2. e = Environment.last # or pick a random environment with deployments
3. e.deployments.success.last.dup.update(finished_at: 1.year.ago)
4. e.reload.last_deployment - should not be the newly created deployment in step 3

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #341840 (closed)