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.
git-svn-id: https://develop.svn.wordpress.org/trunk@48532 602fd350-edb4-49c9-b593-d223f7449a82
