Sitemaps: Rename `wp_get_sitemaps_providers()` to `wp_get_sitemap_providers()`.

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

Props pbiron, swissspidy, SergeyBiryukov.
See #50724.
Built from https://develop.svn.wordpress.org/trunk@48541


git-svn-id: https://core.svn.wordpress.org/trunk@48303 1a063a9b-81f0-0310-95a4-ce76da25c4cd