Update stable branches review on patches and internal releases to only account for release environments

Context

This is a followup issue from #20995 (comment 2518855798) to make stable branch reviews on the patch and internal release pipelines more efficient, and less flakey error-prone.

For patch and internal releases, we suspect that the pre-merge pipeline checks (on the backport MR pipeline) and the post-merge checks (on the MR merge commit pipeline on the stable branches) are the same. (To be corroborated by DexEx on #21002 (closed))

This means that during the post-merge stages of the releases, we could simplify the checks (e.g. security_release_tag:check_component_branch_pipeline_status) to check the status of the release environments pipeline trigger job, and not the entire pipeline's status.

Benefits

• After we work on Improve Release Environment Pipeline Efficiency (#20964 - closed)
  • the branch status check job will no longer need to wait until the end of the entire pipeline, just the release environments downstream pipeline
  • the job will be independent of the flaky specs on the commit pipeline

Feature Flag

stable_branch_check_release_environments_only

If we put the changes under a feature flag, we can continue without being blocked by #21002 (closed)

The FF can be removed once DevEx confirms that the pre-merge and post-merge pipelines should behave the same.

Exit Criteria

• When FF is turned on, the patch release stable branch pipeline status check passes when the trigger jobs start-release-environments-security-pipeline of the stable branches' pipelines are green
• When FF is turned on, the internal release stable branch pipeline status check passes when the trigger jobs start-release-environments-security-pipeline of the internal release stable branches' pipelines are green