Add PEP Scoped Implicit Merge Check

What does this MR do and why?

Enables the security_policy_pipeline_check feature flag by default by removing the feature flag definition file and updating the documentation to reflect that the feature is now generally available.

This MR is part of the rollout for the PEP/SEP Scoped Implicit Merge Check feature. When Pipeline Execution Policies or Scan Execution Policies are configured for a project, this mergeability check requires all pipelines (both branch and MR pipelines) for the latest commit to succeed before the merge request can be merged. This prevents broken code from being merged when customers have duplicate pipelines (branch + MR).

Changes

1. Removed feature flag (config/feature_flags/gitlab_com_derisk/security_policy_pipeline_check.yml): The security_policy_pipeline_check feature flag is removed, enabling the feature by default for all users.

2. Updated documentation (doc/user/application_security/policies/pipeline_execution_policies.md): Removed the feature flag availability notice and updated the history to reflect the feature is now enabled by default in GitLab 18.11.

3. Updated specs (ee/spec/services/merge_requests/mergeability/check_security_policy_pipeline_status_service_spec.rb): Moved the security_policy_pipeline_must_succeed setting disabled context to the top level to ensure the check returns inactive regardless of pipeline status (running, failed, succeeded) when the setting is disabled. This better reflects the behavior now that the feature flag is removed.

Changelog: added EE: true

References

• Related to #589650 (closed)
• Rollout issue: #592205
• Feature flag introduced in !224319 (merged)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.