Sitemaps: Prevent incorrect redirection of paged sitemap requests.

Update `redirect_canonical()` to account for custom pagination and URL format used by sitemaps in order to follow standard practices.

Introduce the function `get_sitemap_url()` to simplify getting the index and provider URLs as needed.

Props jonathanstegall, pbiron, GamerZ, salvoaranzulla, peterwilsoncc.
Merges [48872] to the 5.5 branch.
Fixes #50910.


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