Fix release post blog for small screens

Why is this change being made?

This MR fixes the problem described in #7873 (closed) plus the horizontal scroll "glitch".

See both problems here:

Screen_Recording_2021-11-23_at_6.11.46_PM

Link to the same blog post page in Review app: https://fix-release-post-blog-for-small-screen.about.gitlab-review.app/releases/2020/04/22/gitlab-12-10-released/

And video showing the fixed page in Review app:

Screen_Recording_2021-11-23_at_6.46.24_PM

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
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