[8.14] [BUG][8.12-8.15] Removes the docs for the Last Alert Status Change field and updates the "About" section screenshot (backport #6369)

[mergify]

Removes the docs for the Last Alert Status Change field (which was removed in 8.12 by elastic/kibana#172888) and updates the "About" section screenshot.

Preview: View detection alert details

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This is an automatic backport of pull request #6369 done by [Mergify](https://mergify.com).

[github-actions]

A documentation preview will be available soon.

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