Sitemaps: Rename 'sitemap' to 'provider' in `WP_Sitemaps_Registry` class.

This class is for registering sitemap providers, not individual sitemaps.

Props swissspidy.
Fixes #50724.
Built from https://develop.svn.wordpress.org/trunk@48536


git-svn-id: https://core.svn.wordpress.org/trunk@48298 1a063a9b-81f0-0310-95a4-ce76da25c4cd