Trim project-maintainers.csv to projects not yet migrated to .project and add PR template note about the .project-managed maintainer list

.github/pull_request_template.md

@@ -1,5 +1,10 @@
 # Checklist for maintainer updates
 
+> [!IMPORTANT]
+> **Not all projects are in `project-maintainers.csv`.** If your project isn't listed here, its maintainers are managed automatically and listed in [`project-maintainers-.project.csv`](project-maintainers-.project.csv). To change them, update `maintainers.yaml` in your project's own `.project` repo — the change will be handled by automation. Do not edit `project-maintainers-.project.csv` directly.
+>
+> Access to the CNCF Service Desk and project maintainer mailing lists is controlled by the link between a maintainer's GitHub handle and their LFID, which can be updated at [openprofile.dev](https://openprofile.dev). Your name and affiliation shown in [`project-maintainers-.project.csv`](project-maintainers-.project.csv) are also taken from your LFID.
+
 > [!NOTE]  
 > **Delete this template if you're not changing the CSV file**