A number of locked down installs remove `readme.html` or make it inaccessible which would result in an update failure. Props polevaultweb for the initial patch. Fixes #31420. Built from https://develop.svn.wordpress.org/trunk@38898 git-svn-id: http://core.svn.wordpress.org/trunk@38841 1a063a9b-81f0-0310-95a4-ce76da25c4cd