Updated the frontmatter text and added step for social

Why is this change being made?

Added the new frontmatter info and additional step to mention social team to add custom image to release landing page. We tested this and concepted it in this issue: https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/2609

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies

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