Sitemaps: Correctly enforce maximum number of sitemaps in index.

Before this change, the limit of 50k entries was enforced for the number of providers, not the amount of sitemaps all providers add to the index in total.

Props pbiron, swissspidy.
Fixes #50666.
Built from https://develop.svn.wordpress.org/trunk@48532


git-svn-id: https://core.svn.wordpress.org/trunk@48294 1a063a9b-81f0-0310-95a4-ce76da25c4cd