Administration: Restore the `excerpt` key for the Extended view mode for...

Administration: Restore the `excerpt` key for the Extended view mode for backward compatibility for now.

Props Offereins.
See #49715.
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git-svn-id: https://core.svn.wordpress.org/trunk@48219 1a063a9b-81f0-0310-95a4-ce76da25c4cd