Only use release notes for release post entry

Why is this change being made?

@sarahwaldner reported this problem where it was unclear that only the first block of either Release notes or Problem to solve was used for the description, image and docs link. Instead of making the script more complicated, this MR simplifies the implementation to only use the Release notes section making the behavior more explicit.

Thanks for the feedback @sarahwaldner!

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][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately 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][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.