Disable cluster_management_project by default

As we have some new functionality in the DeploymentPlatform query, we
should validate this on staging at least before enabling to everyone.

app/models/concerns/deployment_platform.rb

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

doc/user/clusters/management_project.md

@@ -95,9 +95,9 @@ configure production cluster:
     name: production
 ```
 
-## Disabling this feature
+## Enabling this feature
 
-This feature is enabled by default. To disable this feature, disable the
+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,