Enable cluster_management_project FF by default

This has been verified on GitLab.com now. But keeping the ability to
disable just in case.

app/models/concerns/deployment_platform.rb

@@ -12,7 +12,7 @@ module DeploymentPlatform
   private
 
   def cluster_management_project_enabled?
-    Feature.enabled?(:cluster_management_project, self)
+    Feature.enabled?(:cluster_management_project, self, default_enabled: true)
   end
 
   def find_deployment_platform(environment)

changelogs/unreleased/make_cluster_management_project_ff_default_enabled.yml

+---
+title: Specify management project for a Kubernetes cluster
+merge_request: 20216
+author:
+type: added

doc/user/clusters/management_project.md

@@ -83,7 +83,7 @@ to a management project:
 | Staging     | `staging`         |
 | Production  | `production`      |
 
-The the following environments set in
+The following environments set in
 [`.gitlab-ci.yml`](../../ci/yaml/README.md) will deploy to the
 Development, Staging, and Production cluster respectively.
 
@@ -109,16 +109,3 @@ configure production cluster:
   environment:
     name: production
 ```
-
-## Enabling this feature
-
-This feature is disabled by default. To enable this feature, enable the
-feature flag `:cluster_management_project`.
-
-To check if the feature flag is enabled on your GitLab instance,
-please ask an administrator to execute the following in a Rails console:
-
-```ruby
-Feature.enabled?(:cluster_management_project)     # Check if it's enabled or not.
-Feature.disable(:cluster_management_project)      # Disable the feature flag.
-```