Remove <atom:description> from Atom feeds

What does this MR do and why?

Remove <atom:description> from Atom feeds.

• <atom:description> isn't a valid Atom element. Added in af33338b (2016).
• <atom:content> was added in 28a52503 (2019), which is the correct element. (Presumably this was done because nothing actually reads <atom:description>!)
• Its content has been double-escaped since 5afc10ad (2021); this has never been raised as a bug, because nothing consumes it!
• The 5 other Atom builders don't include this, because it isn't Atom!

Namespaces

I call them all <atom:blah> (per RFC4287) because the root element opens with this declaration:

<feed xmlns="http://www.w3.org/2005/Atom" xmlns:media="http://search.yahoo.com/mrss/">

The xmlns attribute means every unnamespaced element is implicitly taken from the http://www.w3.org/2005/Atom namespace. An actual unnamespaced <description> tag exists as part of the RSS 2.0 spec, which is presumably how it got mistakenly added originally, but that isn't this.

Screenshots or screen recordings

Feed XML only, since this is never exposed by a reader:

Before	After

How to set up and validate locally

1. Set up an MR in a project on your GDK.
2. Go to the merge requests feed for that project:
   
3. Observe the feed contains a <description> element, which has the raw Markdown input, escaped twice (!). Nothing reads this. The <content type="html> tag contains the correct, escaped-once (for Atom) HTML-rendered content.
4. Check this branch out.
5. Refresh the feed.
6. The <description> element is gone. <content type="html"> correctly remains.

MR acceptance checklist

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