Note as-if-foss-run-once behaviour in pipeline tier transition message

What does this MR do and why?

We had another conversation about "FOSS master broken due to ~"pipeline:as-if-foss-run-once"" in #s_developer_experience today, when @jay_mccure suggested:

I think we can add a sentence to this message gitlab-org/gitlab!219133 (comment 3052740521) to make it clear that if changes are added after the one FOSS pipeline has been run they won't get tested in a FOSS pipeline unless you add the run-against-foss label if you want subsequent FOSS runs

This MR implements that! It also renames a confusingly-named constant to match its label (the label name got changed in the past), so these two similar labels don't get confused.

Expected impact & dry-runs

I expect the message will have the new note added. It looks like this rendered:

Note: The "as-if-FOSS" pipeline only runs once per merge request. After it succeeds, the ~"pipeline:as-if-foss-run-once" label is applied and subsequent pushes will not trigger another FOSS pipeline. If you push changes that could affect FOSS compatibility, apply the pipeline:run-as-if-foss label and run a new pipeline.

Note that ~"pipeline:as-if-foss-run-once" doesn't render here in triage-ops, since it doesn't exist on this project.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review