Remove deploy_boards_dedupe_instances feature flag

Rollout bug fix to self managed instances
Update documentation and specs

config/feature_flags/development/deploy_boards_dedupe_instances.yml

----
-name: deploy_boards_dedupe_instances
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40768
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/258214
-type: development
-group: group::progressive delivery
-default_enabled: false

doc/user/project/deploy_boards.md

@@ -41,9 +41,9 @@ knowledge. In particular, you should be familiar with:
 - [Kubernetes namespaces](https://kubernetes.io/docs/concepts/overview/working-with-objects/namespaces/)
 - [Kubernetes canary deployments](https://kubernetes.io/docs/concepts/cluster-administration/manage-deployment/#canary-deployments)
 
-NOTE: **Note:**
-Apps that consist of multiple deployments are shown as duplicates on the deploy board.
-Follow [this issue](https://gitlab.com/gitlab-org/gitlab/-/issues/8463) for details.
+In GitLab 13.5 and earlier, apps that consist of multiple deployments are shown as
+duplicates on the deploy board. This is [fixed](https://gitlab.com/gitlab-org/gitlab/-/issues/8463)
+in GitLab 13.6.
 
 ## Use cases
 

ee/changelogs/unreleased/fix-deploy-board-multi-deployment-pods.yml

+---
+title: Fix duplicate instances in deploy boards when multiple deployments have the same track
+merge_request: 46374
+author:
+type: fixed

ee/lib/gitlab/kubernetes/rollout_status.rb

@@ -54,13 +54,7 @@ module Gitlab
       def initialize(deployments, pods: [], ingresses: [], status: :found)
         @status       = status
         @deployments  = deployments
-
-        @instances = if ::Feature.enabled?(:deploy_boards_dedupe_instances)
-                       RolloutInstances.new(deployments, pods).pod_instances
-                     else
-                       deployments.flat_map(&:instances)
-                     end
-
+        @instances = RolloutInstances.new(deployments, pods).pod_instances
         @canary_ingress = ingresses.find(&:canary?)
 
         @completion =