Revise release post header to have same classes for all posts

Why is this change being made?

The posts no longer need to distinguish between a dark or light cover image as they are now all the same configuration.

Posts without dark-header aren't showing the gradient correctly:

• Incorrect
• Correct

This MR will style the header so that they all appear correctly.

Posts for review

• 15.7
• 15.6
• 15.5
• 15.4
• 15.3
• 15.2
• 15.1
• 15.0
• 14.10
• 14.9
• 14.8
• 14.7
• 14.6
• 14.5
• 14.4
• 14.3
• 14.2
• 14.1
• 14.0

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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• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • 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
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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