Update metadata association

Update metadata to link to Release Management

doc/user/project/pages/lets_encrypt_for_gitlab_pages.md

 ---
-stage: none
-group: unassigned
+stage: Release
+group: Release Management
 info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
 description: "How to secure GitLab Pages websites with Let's Encrypt (manual process, deprecated)."
 ---