DRAFT: Update release post cover images on 15.8+ release posts

Why is this change being made?

Related to: Product#5162

This MR will update the cover image on all 15.* release posts to align with the new branding.

• Replaces cover images with an on-brand look
• Updates front matter of posts to for correct layout in the release post template

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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  • The when to get approval handbook section explains the workflow in more detail
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