Sitemaps: Rename `wp_register_sitemap()` to `wp_register_sitemap_provider()`.

Follow-up to [48536], [48540], [48541].

Props pbiron.
Fixes #50724.
Built from https://develop.svn.wordpress.org/trunk@48553


git-svn-id: https://core.svn.wordpress.org/trunk@48315 1a063a9b-81f0-0310-95a4-ce76da25c4cd