Enable `ci_plan_needs_size_limit` feature flag

- Add feature flag definition
- Mark it as default enabled

config/feature_flags/development/ci_plan_needs_size_limit.yml

+---
+name: ci_plan_needs_size_limit
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37568
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/238173
+group: group::ci
+type: development
+default_enabled: true
\ No newline at end of file

doc/ci/yaml/README.md

@@ -1989,9 +1989,7 @@ This example creates four paths of execution:
 - The maximum number of jobs that a single job can need in the `needs:` array is limited:
   - For GitLab.com, the limit is ten. For more information, see our
     [infrastructure issue](https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/7541).
-  - For self-managed instances, the limit is:
-    - 10, if the `ci_plan_needs_size_limit` feature flag is disabled (default).
-    - 50, if the `ci_plan_needs_size_limit` feature flag is enabled. This limit [can be changed](#changing-the-needs-job-limit-core-only).
+  - For self-managed instances, the limit is: 50. This limit [can be changed](#changing-the-needs-job-limit-core-only).
 - If `needs:` refers to a job that is marked as `parallel:`.
   the current job will depend on all parallel jobs created.
 - `needs:` is similar to `dependencies:` in that it needs to use jobs from prior stages,
@@ -2002,18 +2000,10 @@ This example creates four paths of execution:
 
 ##### Changing the `needs:` job limit **(CORE ONLY)**
 
-The maximum number of jobs that can be defined within `needs:` defaults to 10.
+The maximum number of jobs that can be defined within `needs:` defaults to 50.
 
-To change this limit to 50 on a self-managed installation, a GitLab administrator
-with [access to the GitLab Rails console](../../administration/feature_flags.md)
-can enable the `:ci_plan_needs_size_limit` feature flag:
-
-```ruby
-Feature::enable(:ci_plan_needs_size_limit)
-```
-
-After the feature flag is enabled, you can choose a custom limit. For example, to
-set the limit to 100:
+A GitLab administrator with [access to the GitLab Rails console](../../administration/feature_flags.md)
+can choose a custom limit. For example, to set the limit to 100:
 
 ```ruby
 Plan.default.actual_limits.update!(ci_needs_size_limit: 100)

lib/gitlab/ci/features.rb

@@ -65,7 +65,7 @@ module Gitlab
       end
 
       def self.ci_plan_needs_size_limit?(project)
-        ::Feature.enabled?(:ci_plan_needs_size_limit, project)
+        ::Feature.enabled?(:ci_plan_needs_size_limit, project, default_enabled: true)
       end
 
       def self.job_entry_matches_all_keys?