Fix EnvironmentSerializer preloads unrelated pipelines/builds

What does this MR do?

Reported at !58748 (comment 562639919) by @ahegyi . Currently, accessing https://gitlab.com/gitlab-org/gitlab-docs/-/environments causes timeout by a massive preloading on unrelated pipelines/builds records.

The culprit

It seems we're preloading all deployments (and pipelines/builds) of each environment, where only the lastest/upcoming deployment is necessary. For example, currently the AR relationship looks like the following:

class Environment < ApplicationRecord
  has_one :last_deployment, -> { success.order('deployments.id DESC') }, class_name: 'Deployment', inverse_of: :environment
  has_one :upcoming_deployment, -> { running.order('deployments.id DESC') }, class_name: 'Deployment', inverse_of: :environment

and when we execute environments.preload(:last_deployment), it generates the following query:

SELECT "deployments".*
FROM "deployments"
WHERE "deployments"."status" = $1
  AND "deployments"."environment_id" IN ($2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21)
ORDER BY deployments.id DESC

So the preloader doesn't have idea which deployment the latest per environment, so that loading associated pipelines/builds of all deployments. We need a grouping clause here.

Fortunately, we have Deployment.distinct_on_environment scope that was introduced for a similar problem in environment dashboard context. With it, the AR relationship should be:

class Environment < ApplicationRecord
  has_one :last_deployment, -> { success.distinct_on_environment }, class_name: 'Deployment', inverse_of: :environment
  has_one :upcoming_deployment, -> { running.distinct_on_environment }, class_name: 'Deployment', inverse_of: :environment

which generates the following query:

SELECT DISTINCT ON (environment_id) deployments.*
FROM "deployments"
WHERE "deployments"."status" = $1
  AND "deployments"."environment_id" IN ($2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16, $17, $18, $19, $20, $21)
ORDER BY environment_id, deployments.id DESC 

This correctly fetches the latest/upcoming deployment per environment. We have confirmed that only relevant pipelines/builds are preloaded for them.

SELECT "ci_pipelines".* FROM "ci_pipelines" WHERE "ci_pipelines"."id" IN ($1, $2) 

Related !58748 (merged)

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