Fix top feature markup (13.7)

Why is this change being made?

I noticed that the top feature of the 13.7 release post wasn't centered in the blog post so I tried to figure out why.

Found out that the feature was tagged as primary, not top.

Notes that the handbook is inconsistent:

• Here it mentions the top feature markup: https://about.gitlab.com/handbook/marketing/blog/release-posts/#release-post-top-feature
• Here it doesn't: https://about.gitlab.com/handbook/marketing/blog/release-posts/#top-feature

I wonder if we need 2 sections for each? 🤔

Let's fix the markup on 13.7 first and make the handbook update in another MR.

Author Checklist

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