Use `CI_OPEN_MERGE_REQUESTS` environment variable

- This changes `ci_pipeline_open_merge_requests` to
  be on by default

config/feature_flags/development/ci_pipeline_open_merge_requests.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/292727
 milestone: '13.7'
 group: group::memory
 type: development
-default_enabled: false
+default_enabled: true

doc/ci/variables/predefined_variables.md

@@ -66,7 +66,7 @@ Kubernetes-specific environment variables are detailed in the
 | `CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_NAME` | 12.3   | all    | The target branch name of the pull request if [the pipelines are for external pull requests](../ci_cd_for_external_repos/index.md#pipelines-for-external-pull-requests). Available only if `only: [external_pull_requests]` or [`rules`](../yaml/README.md#rules) syntax is used and the pull request is open. |
 | `CI_EXTERNAL_PULL_REQUEST_TARGET_BRANCH_SHA`  | 12.3   | all    | The HEAD SHA of the target branch of the pull request if [the pipelines are for external pull requests](../ci_cd_for_external_repos/index.md#pipelines-for-external-pull-requests). Available only if `only: [external_pull_requests]` or [`rules`](../yaml/README.md#rules) syntax is used and the pull request is open. |
 | `CI_HAS_OPEN_REQUIREMENTS`                    | 13.1   | all    | Included with the value `true` only if the pipeline's project has any open [requirements](../../user/project/requirements/index.md). Not included if there are no open requirements for the pipeline's project. |
-| `CI_OPEN_MERGE_REQUESTS`                      | 13.7   | all    | Contains a comma-delimited list of up to 4 Merge Requests from the current source project and branch in the form `gitlab-org/gitlab!333,gitlab-org/gitlab-foss!11`. |
+| `CI_OPEN_MERGE_REQUESTS`                      | 13.8   | all    | Available in branch and merge request pipelines. Contains a comma-separated list of up to four merge requests that use the current branch and project as the merge request source. For example `gitlab-org/gitlab!333,gitlab-org/gitlab-foss!11`.                                                                                                  |
 | `CI_JOB_ID`                                   | 9.0    | all    | The unique ID of the current job that GitLab CI/CD uses internally. |
 | `CI_JOB_IMAGE`                                | 12.9   | 12.9   | The name of the image running the CI job. |
 | `CI_JOB_MANUAL`                               | 8.12   | all    | The flag to indicate that job was manually started. |

lib/gitlab/ci/features.rb

@@ -56,7 +56,7 @@ module Gitlab
       end
 
       def self.pipeline_open_merge_requests?(project)
-        ::Feature.enabled?(:ci_pipeline_open_merge_requests, project, default_enabled: false)
+        ::Feature.enabled?(:ci_pipeline_open_merge_requests, project, default_enabled: true)
       end
 
       def self.ci_pipeline_editor_page_enabled?(project)