Canonical: Redirect paged requests for non-paginated posts to the post permalink.

This avoids displaying duplicate content of the same post under different URLs and ensures the canonical URL is correct.

Previously, requests for invalid page numbers were only redirected to the post permalink if the post was actually paginated using the `<!--nextpage-->` marker.

Follow-up to [34492].

Props jeremyfelt, prografika, sachit.tandukar, subrataemfluence, hronak, ekatherine, henry.wright, chesio, dd32, SergeyBiryukov.
Fixes #40773. See #45337, #28081, #11694.
Built from https://develop.svn.wordpress.org/trunk@47727


git-svn-id: https://core.svn.wordpress.org/trunk@47504 1a063a9b-81f0-0310-95a4-ce76da25c4cd