DOC-1793 add time-based retention to beta what's new

[micheleRP]

Description

This PR adds a blurb to the What's New for the new time-based retention validation added in #1429

Resolves https://redpandadata.atlassian.net/browse/DOC-1793
Review deadline:

Page previews

What's New

Checks

• New feature
• Content gap
• Support Follow-up
• Small fix (typos, links, copyedits, etc)

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch DOC-1793-add-time-based-retention-to-beta-what's-new

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[Feediver1]

lgtm